Optical scanner and imaging apparatus using the same

ABSTRACT

An optical scanner includes a light source modulated based on image data, an optical deflection and scanning part deflecting a light beam emitted from the light source, and a scanning and imaging optical system condensing the deflected light beam toward a scanning surface so as to form a light spot optically scanning the scanning surface. The effective scanning region of the scanning surface is divided into a plurality of regions according to a scanning line curving characteristic. Suitable image data for optically scanning the divided regions are selected from image data of a plurality of image lines every time the light spot optically scans the effective scanning region, so that the image data of each of the image lines is written with scanning line curving being corrected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to optical scanners and imagingapparatuses, and more particularly to an optical scanner that forms alight spot on a scanning surface by using optical deflection andscanning means and a scanning and imaging optical system and opticallyscans the scanning surface with the light spot, and an imaging apparatususing such an optical scanner.

2. Description of the Related Art

Optical scanners that deflect a light beam emitted from a light sourcemodulated based on image data in a main (primary) scanning directionusing optical deflection and scanning means such as a rotary polygonalmirror, focus the deflected light beam into a light spot on a scanningsurface (surface to be scanned) using a scanning and imaging opticalsystem such as an fθ lens, and scan the scanning surface with the lightspot formed thereon are widely known in relation to imaging apparatusessuch as optical printers, optical plotters, and digital copiers.

In an imaging apparatus using such an optical scanner, generally, animaging process including a step of writing an image by optical scanningis performed. The quality of the formed image depends on the quality ofthe optical scanning. The optical scanning quality depends on thescanning characteristics of the optical scanner in the main and subscanning directions.

The scanning characteristics in the main scanning direction include theconstant velocity characteristic of optical scanning.

In the case of using a rotary polygonal mirror as optical deflection andscanning means, for instance, a light beam is deflected at a constantangular velocity. Therefore, a scanning and imaging optical systemhaving the fθ characteristic is employed in order to realize theconstant velocity characteristic of optical scanning. It is not easy,however, to realize a perfect constant velocity characteristic ofoptical scanning due to other characteristics required of the scanningand imaging optical system. Therefore, in actual optical scanning,optical scanning is not performed at a perfect constant velocity, sothat the constant velocity characteristic as a scanning characteristicis accompanied by “deviation or difference from ideal constant velocityscanning.”

The scanning characteristics in the main scanning direction aregenerated not only by the above-described insufficient correction of thefθ characteristic, but also by: a decrease in the accuracy of opticalcomponents or the accuracy of attachment to the housing of the opticalscanning system; changes in the optical component properties caused byenvironmental changes such as changes in temperature and humidity insidethe optical scanner; and the shift of a photosensitive body or anintermediate transfer body, which substantially forms the scanningsurface, in the direction of its rotation axis.

The scanning characteristics in the sub scanning direction include“scanning line curving” and “scanning line tilting.” A scanning line isthe trace of movement of a light spot on a scanning surface, andideally, is a straight line. The optical scanner is designed so as tohave a straight scanning line. As it is, however, it is common that thescanning line is curved due to processing or assembly error. Thiscurving of the scanning line is referred to as “scanning line curving,”and the degree of curving is referred to as the “scanning line curvingcharacteristic.”

In the case of using an “imaging mirror” as a scanning and imagingoptical system so as to form an angle in the sub scanning directionbetween the direction in which the deflected light beam is made incidenton the imaging mirror and the direction in which the deflected lightbeam is reflected therefrom, scanning line curving occurs in principle.In the case of forming the scanning and imaging optical system as a lenssystem, scanning line curving is unavoidable in multi-beam scanningwhere the scanning surface is optically scanned by a plurality of lightspots separated in the sub scanning direction.

The “scanning line tilting,” which is a phenomenon where the scanningline does not cross the sub scanning direction correctly at rightangles, is a type of scanning line curving. Accordingly, in thefollowing description, “scanning line tilting” is included in “scanningline curving” unless otherwise indicated.

Imperfection of the constant velocity characteristic of optical scanningdistorts a formed image in the main scanning direction, and scanningline curving distorts a formed image in the sub scanning direction. Whena single optical scanner writes and forms a monochrome image, the formedimage includes no “visually recognizable distortion” if scanning linecurving and imperfection of the constant velocity characteristic (or thedeviation or difference from the ideal constant velocity scanning) aresuppressed to a sufficient extent. However, the image is better withless distortion even if the distortion is visually unrecognizable.

Even in a monochrome image, however, scanning line curving becomes anunignorable problem if image linearity is considered important as in thecase of a CAD (computer-aided design) output because even a slight curveof the scanning line significantly degrades the quality of the outputimage.

Conventionally, in color copiers, images of three colors of magenta,cyan, and yellow or four colors of those three colors plus black areformed as color component images, and a composite color image is formedby superimposing those color component images.

In forming such a color image, a so-called “tandem-type” imaging methodis employable. In this imaging method, color component images are formedon different photosensitive bodies by different optical scanners (ordifferent optical scanner parts), respectively. According to thisimaging method, however, if the degree of scanning line curving ortilting differs between the optical scanners, an abnormality called“color misregistration” where colors are superimposed inappropriately ontop of each other is caused to appear in the formed color image by aslight misalignment of the scanning lines of the optical scanners,thereby degrading the quality of the color image, even if the scanningline curving or tilting is corrected to an acceptable level in eachoptical scanner.

Further, the “color misregistration” may appear as a phenomenon where adesired tone cannot be obtained in the color image.

In recent years, a special surface, typically, an aspheric surface, hasbeen commonly employed in the imaging optical system of the opticalscanners in a bid to improve the scanning characteristics. Accordingly,a low-cost imaging optical system of a resin material, with which such aspecial surface can be formed easily, is widely used.

When the resin imaging optical system is affected by changes intemperature or humidity, the shape and the refractive index of itselement are subject to change. Therefore, the optical properties of theimaging optical system are subject to change. These changes in theoptical properties also change the scanning line curving characteristicand the constant velocity characteristic. Then, in the case ofsuccessively forming tens of color images, for instance, the imagingapparatus operates continuously to increase its internal temperature, sothat the optical properties of the imaging optical system change togradually change the scanning line curving characteristic and theconstant velocity characteristic of each optical scanner. This may causethe phenomenon of color misregistration, thus resulting in a great tonedifference between a color image obtained at the beginning and a colorimage obtained at the end.

Japanese Laid-Open Patent Application No. 2001-215437 discloses atechnology of forming an exposure distribution in the sub scanningdirection on a scanning surface or in a scanning region. JapaneseLaid-Open Patent Application No. 2000-235290 discloses a technology ofadjusting a scanning line position in the sub scanning direction bycontrolling the modulation timing of a light source.

Further, Japanese Laid-Open Patent Application No. 8-313941 discloses atechnology relating to a liquid crystal deflection element.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providean optical scanner and an imaging apparatus in which the above-describeddisadvantages are eliminated.

A more specific object of the present invention is to provide an opticalscanner that effectively corrects scanning line curving so that goodoptical scanning can be performed.

Another more specific object of the present invention is to provide anoptical scanner that suitably corrects, as well as scanning linecurving, a deviation of a scanning position in the main scanningdirection resulting from the constant velocity characteristic so thatbetter optical scanning can be performed.

Another more specific object of the present invention is to provide animaging apparatus using any of such optical scanners so that anexcellent image can be formed through optical scanning.

Yet another more specific object of the present invention is to correctthe deviation, curve, and tilt of a scanning line in one scanning regionrelative to a corresponding scanning line in another scanning region inthe case of scanning a plurality of scanning regions independently ofone another.

The above objects of the present invention are achieved by an opticalscanner including: a light source modulated based on image data; anoptical deflection and scanning part deflecting a light beam emittedfrom the light source; and a scanning and imaging optical systemcondensing the deflected light beam toward a scanning surface so as toform a light spot thereon, the light spot optically scanning thescanning surface, wherein an effective scanning region of the scanningsurface is divided into a plurality of regions according to a scanningline curving characteristic, and suitable image data for opticallyscanning the divided regions are selected from image data of a pluralityof image lines every time the light spot optically scans the effectivescanning region, so that the image data of each of the image lines iswritten with scanning line curving being corrected.

The above objects of the present invention are also achieved by anoptical scanner including: a light source modulated based on image datato emit a plurality of light beams; an optical deflection and scanningpart deflecting the light beams; and a scanning and imaging opticalsystem condensing the deflected light beams toward a scanning surface sothat light spots are formed on the scanning surface so as to beseparated from each other in a sub scanning direction, the light spotshaving substantially equal scanning line curving characteristics andoptically scanning the scanning surface simultaneously, wherein aneffective scanning region of the scanning surface is divided into aplurality of regions according to the scanning line curvingcharacteristics, and a suitable one of the light beams is selected foroptical scanning in each of the divided regions with respect to each ofimage lines every time the light spots optically scan the effectivescanning region simultaneously, so that the image data of each of theimage lines is written with scanning line curving being corrected.

The above objects of the present invention are also achieved by animaging apparatus including: N (≧2) photoconductive photosensitivebodies arranged along a path for conveying a sheet-like recordingmedium; and N optical scanners optically scanning the N photosensitivebodies, respectively, so as to write and form different electrostaticlatent images thereon, the electrostatic latent images being madevisible as toner images of different colors so that the toner images aretransferred to and fixed on the sheet-like recording medium, beingsuperimposed one over another, wherein one of the N photosensitivebodies has a black toner image formed thereon, and each of the opticalscanners for the remaining N-1 photosensitive bodies includes: a lightsource modulated based on image data; an optical deflection and scanningpart deflecting a light beam emitted from the light source; and ascanning and imaging optical system condensing the deflected light beamtoward a scanning surface of the corresponding photosensitive body so asto form a light spot on the scanning surface, the light spot opticallyscanning the scanning surface, an effective scanning region of thescanning surface being divided into a plurality of regions according toa scanning line curving characteristic, and suitable image data foroptically scanning the divided regions being selected from image data ofa plurality of image lines every time the light spot optically scans theeffective scanning region, so that the image data of each of the imagelines is written with scanning line curving being corrected based on theblack toner image.

The above objects of the present invention are also achieved by animaging apparatus including: N (≧2) photoconductive photosensitivebodies arranged along a path for conveying a sheet-like recordingmedium; and N optical scanners optically scanning the N photosensitivebodies, respectively, so as to write and form different electrostaticlatent images thereon, the electrostatic latent images being madevisible as toner images of different colors so that the toner images aretransferred to and fixed on the sheet-like recording medium, beingsuperimposed one over another, wherein one of the N photosensitivebodies has a black toner image formed thereon, and each of the opticalscanners for the remaining N-1 photosensitive bodies includes: a lightsource modulated based on image data to emit a plurality of light beams;an optical deflection and scanning part deflecting the light beams; anda scanning and imaging optical system condensing the deflected lightbeams toward a scanning surface of the corresponding photosensitive bodyso that light spots are formed on the scanning surface so as to beseparated from each other in a sub scanning direction, the light spotshaving substantially equal scanning line curving characteristics andoptically scanning the scanning surface simultaneously, an effectivescanning region of the scanning surface being divided into a pluralityof regions according to the scanning line curving characteristics, and asuitable one of the light beams being selected for optical scanning ineach of the divided regions with respect to each of image lines everytime the light spots optically scan the effective scanning regionsimultaneously, so that the image data of each of the image lines iswritten with scanning line curving being corrected based on the blacktoner image.

The above objects of the present invention are also achieved by anoptical scanner including a plurality of optical scanning partsoptically scanning a plurality of scanning regions, respectively, so asto write images to form a desired image separately in the scanningregions, the optical scanning parts each deflecting a light beam emittedfrom a modulated light source by an optical deflection and scanning partso that the deflected light beam is guided by a scanning and imagingoptical system to form a light spot in the corresponding scanningregion, the light spot optically scanning the corresponding scanningregion, wherein, in at least one of the scanning regions, an exposuredistribution is formed to have a center of gravity between scanninglines adjacent in a sub scanning direction so that a position of thecenter of gravity in the sub scanning direction is shiftable.

The above objects of the present invention are also achieved by animaging apparatus including: a plurality of photoconductivephotosensitive bodies as scanning regions; and an optical scanneroptically scanning the scanning regions so as to form electrostaticlatent images therein, the electrostatic latent images being madevisible with toners of different colors as toner images, the tonerimages being transferred to a sheet-like recording medium superimposedone over another so as to form a desired image, wherein the opticalscanner includes a plurality of optical scanning parts opticallyscanning the scanning regions, respectively, so as to write theelectrostatic latent images to form the desired image separately in thescanning regions, the optical scanning parts each deflecting a lightbeam emitted from a modulated light source by an optical deflection andscanning part so that the deflected light beam is guided by a scanningand imaging optical system to form a light spot in the correspondingscanning region, the light spot optically scanning the correspondingscanning region, and an exposure distribution is formed to have a centerof gravity between scanning lines adjacent in a sub scanning directionin at least one of the scanning regions so that a position of the centerof gravity in the sub scanning direction is shiftable.

The above objects of the present invention are also achieved by anoptical scanner including an optical scanning part optically scanning ascanning region so as to write a desired image therein, the opticalscanning part deflecting a light beam emitted from a modulated lightsource by an optical deflection and scanning part so that the deflectedlight beam is guided by a scanning and imaging optical system to form alight spot in the scanning region, the light spot optically scanning thescanning region, wherein an exposure distribution is formed to have acenter of gravity between scanning lines adjacent in a sub scanningdirection in the scanning region so that a position of the center ofgravity in the sub scanning direction is shiftable.

The above objects of the present invention are further achieved by animaging apparatus including: a photoconductive photosensitive body as ascanning region; and an optical scanner optically scanning the scanningregion so as to form an electrostatic latent image therein, theelectrostatic latent image being made visible with toner as a tonerimage, the toner image being transferred to a sheet-like recordingmedium so as to form a desired image, wherein the optical scannerincludes an optical scanning part optically scanning the scanning regionso as to write the desired image therein, the optical scanning partdeflecting a light beam emitted from a modulated light source by anoptical deflection and scanning part so that the deflected light beam isguided by a scanning and imaging optical system to form a light spot inthe scanning region, the light spot optically scanning the scanningregion, and an exposure distribution is formed to have a center ofgravity between scanning lines adjacent in a sub scanning direction inthe scanning region so that a position of the center of gravity in thesub scanning direction is shiftable.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram for illustrating a divided writing method accordingto the present invention;

FIGS. 2A and 2B are diagrams for illustrating an imaging apparatusaccording to a first embodiment of the present invention;

FIGS. 3A through 3D are diagrams for illustrating a method of correctionfor fine adjustment according to the first embodiment of the presentinvention;

FIG. 4 is a diagram showing a liquid crystal deflection element used forcorrection for fine adjustment according to the first embodiment of thepresent invention;

FIGS. 5A and 5B are diagrams showing optical scanners performingcorrection for fine adjustment according to the first embodiment of thepresent invention;

FIGS. 6A and 6B are diagrams showing typical deflection mirrors, andFIGS. 6C and 6D are diagrams for illustrating configurations forcorrection for fine adjustment using the deflection mirrors according tothe first embodiment of the present invention;

FIGS. 7A and 7B are diagrams for illustrating adjustment of the positionof a light spot in the main scanning direction according to the firstembodiment of the present invention;

FIGS. 8A and 8B are diagrams for illustrating a scanning positiondetecting part employed in the optical scanner according to the firstembodiment of the present invention;

FIGS. 9A and 9B are diagrams showing configurations for detecting ascanning position using the scanning position detecting part of FIGS. 8Aand 8B, and FIGS. 9C and 9D are diagrams showing other configurationsfor detecting a scanning position according to the first embodiment ofthe present invention;

FIG. 10 is a diagram for illustrating a light-selective writing methodemployed in an optical scanner according to a second embodiment of thepresent invention;

FIG. 11 is a diagram showing an imaging apparatus according to a thirdembodiment of the present invention;

FIGS. 12A through 12E are diagrams for illustrating correction ofscanning line curving by an optical scanner according to a fourthembodiment of the present invention;

FIG. 13 is a diagram for illustrating correction of scanning linetilting by the optical scanner according to the fourth embodiment of thepresent invention;

FIGS. 14A through 14E are diagrams for illustrating correction ofscanning line curving using three or more light spots according to thefourth embodiment of the present invention;

FIG. 15 is a diagram showing an optical scanner according to a fifthembodiment of the present invention; and

FIGS. 16A and 16B are diagrams for illustrating correction of a scanningline position according to the fifth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to the accompanyingdrawings, of embodiments of the present invention.

First, a description will be given of the definitions of the followingterms employed in the following description.

“Image data” refers to a group of pixel data for an image to be formedby optical scanning. The intensity of a light beam emitted from a lightsource is modulated in accordance with the image data, so that the imageis written to the positions corresponding to the pixels on a scanningsurface. The image data may be obtained by reading a document to beduplicated and converting the document into pixel data. Alternatively,the image data may be generated in a computer, a word processor, adigital camera, or an information terminal device. Further, the imagedata may be transferred as external data.

A “deflection and scanning part” deflects a light beam from a lightsource in the main scanning direction for optical scanning. Aconventional well-known mirror such as a polygon mirror, a pyramidalmirror, a rotary single-surface mirror such as a tenon-shaped mirror, arotary dual-surface mirror, or a galvano mirror may be used as thedeflection and scanning part.

A “scanning and imaging optical system” condenses a light beam deflectedby the deflection and scanning part toward a scanning surface and formsa light spot thereon. The scanning and imaging optical system may beformed as a lens system such as an fθ lens, an imaging mirror systemsuch as an fθ mirror, or a combination of a lens system and a mirrorsystem.

The scanning and imaging optical system may include a resin imagingelement. In this case, the scanning and imaging optical system may beformed partly of resin imaging elements, or the entire scanning andimaging optical system may be formed of resin imaging elements. Theentire scanning and imaging optical system may be formed of a singleresin imaging element.

The scanning and imaging optical system may have the function of makingconstant the velocity of optical scanning by a light spot. When theoptical deflection and scanning part deflects a light beam at constantangular velocity, for instance, a scanning and imaging optical systemhaving the fθ function for causing the light spot to perform opticalscanning at constant velocity is employed. In this case, the fθcharacteristic is the constant velocity characteristic of opticalscanning.

A “scanning surface” refers to a surface optically scanned by a lightspot. Generally, the scanning surface is substantially a photosensitivebody.

A variety of “photosensitive media” may be used. A silver film, forinstance, may be used as a photosensitive medium. In this case, a latentimage formed by writing by optical scanning may be made visible by anormal silver photography process. An imaging apparatus using such aphotosensitive medium may be realized as an optical plate-making deviceor an optical drawing device that draws a CT scan image.

A color-developing medium that develops color due to the thermal energyof a light spot at the time of optical scanning may also be employed asa photosensitive medium. In this case, a visible image can be formeddirectly by optical scanning.

Further, a photoconductive photosensitive body is also employable as aphotosensitive medium. The photoconductive photosensitive body may havea sheet-like shape as zinc oxide paper. The photoconductivephotosensitive body may have a drum-like or belt-like shape and berepeatedly used as a selenium photosensitive body or an organic photoconductor.

An “effective scanning region” refers to a region in the main scanningdirection on the scanning surface optically scanned by a light spot inwhich region the image data is effectively written.

An “image line” refers to an ideal straight line forming one line of animage. A series of data forming the image of the image line is writtento the image line.

First Embodiment

FIG. 1 is a diagram for illustrating a divided writing method accordingto the present invention. According to the divided writing method, whenone light spot performs optical scanning moving on a scanning surface,in each predetermined part of a scanning line, which is the trace ofmovement of the light spot, image data for the image line (correspondingto an ideal straight scanning line) closest to the predetermined part iswritten. That is, every time a light spot optically scans a scanningsurface, image data for a plurality of image lines are selected. Thus,each of the image lines is partially written by each optical scan.Accordingly, the image data of each entire image line are written by aplurality of optical scans by the light spot. According to this method,image data for a plurality of image lines are divided and written duringa single optical scan by a light spot. Further, image data forming oneimage line is written by a plurality of optical scans by the light spot.Therefore, this method is referred to as the divided writing method.FIG. 1 shows a scanning surface (surface to be scanned) on which opticalscanning is performed. In FIG. 1, the upward direction is the subscanning direction. That is, as optical scanning is repeated in the mainscanning direction, a photosensitive medium substantially forming thescanning surface moves in the upward direction of FIG. 1. In thefollowing description, the sub scanning direction may also include thedownward direction in FIG. 1, that is, the direction opposite to thedirection in which the photosensitive medium moves.

In FIG. 1, the reference numerals S1 through S5 denote five “imagelines” spaced in the sub scanning direction. The image lines correspondto ideal straight scanning lines. The reference numerals SL1 through SL5denote virtual scanning lines that are formed on the scanning surface byfive optical scans successively performed by a single light spot. Thatis, the light spot performs the optical scans in the order of SL1, SL2,SL3, SL4, and SL5.

As previously described, an ideal scanning line is straight. If an idealstraight scanning line could be formed, optical scanning in which eachscanning line matches its corresponding image line would be realized. Asit is, however, “scanning line curving” occurs in the actual opticalscanning as previously described.

In the divided writing method, the effective scanning region is dividedinto a plurality of regions Di (i=1, 2, . . . ) as shown in FIG. 1. Inthe case of FIG. 1, the effective scanning region is divided into sixregions D1 through D6. Division of the effective scanning region isdetermined based on the scanning line curving characteristic, that is,the degree of scanning line curving.

In the case of the scanning line SL3, for instance, as optical scanningproceeds from the starting side (from left to right in the drawing), thescanning line SL3 comes closest to: the image line S5 in the region D1;the image line S4 in the region D2; the image line S3 in the region D3;the image line S2 in the region D4; the image line S3 in the region D5;and the image line S4 in the region D6.

Accordingly, as the light spot performs writing along the scanning lineSL3, the light spot writes image data for: the image line S5 in theregion D1; the image line S4 in the region D2; the image line S3 in theregion D3; the image line S2 in the region D4; the image line S3 in theregion D5; and the image line S4 in the region D6.

That is, the image data is marked with the number k of the image lineSk, the number i of the region Di on which writing is to be performed,and the number j of the scanning line SLj to write data for the regionDi. Accordingly, the image data to be written in the region Di of theimage line Sk by the scanning line SLj is indicated as D_(kij). As aspecific example, the above-described case of optical scanning by thescanning line SL3 is considered herein. The index j specifying thescanning line is set to “3.” The image line number k is set to 1 through5 so as to correspond to the image lines S1 through S5.

When the light spot performs optical scanning along the scanning lineSL3 as previously described, for instance, the light intensity of thelight spot drawing the scanning line SL3 is modulated by: image dataD₅₁₃ in the region D1; image data D₄₂₃ in the region D2; image data D₃₃₃in the region D3; image data D₂₄₃ in the region D4; image data D₃₅₃ inthe region D5; and image data D₄₆₃ in the region D6.

FIG. 1 shows, in its lower part, a written image equivalent to one imageline, whose image data is written, so as to correspond to the image lineS3, by optical scanning in which scanning line curving is corrected. Ineach of the regions D1 through D5, this image is written by thecorresponding one of the scanning lines SL1 through SL4.

That is, the image data for writing the image line S3 is divided into:image data D₃₁₁ in the region D1; image data D₃₂₂ in the region D2; theimage data D₃₃₃ in the region D3; image data D₃₄₄ in the region D4; theimage data D₃₅₃ in the region D5; and image data D₃₆₂ in the region D6.Thus, the image data is written by: the scanning line SL1 in the regionD1; the scanning line SL2 in the region D2; the scanning line SL3 in theregion D3; the scanning line SL4 in the region D4; the scanning line SL3in the region D5; and the scanning line SL2 in the region D6.

The written image corresponding to the image line S3 shown in the lowerpart of FIG. 1, which image is formed by writing a straight line in themain scanning direction according to the above-described divided writingmethod, shows discontinuity and “variation” over the divided regions D1through D6. However, the image only varies within the rangecorresponding to the interval between the adjacent image lines, that is,the interval between the image lines SL2 and SL3 or the image lines SL3and SL4. This interval is referred to as one scanning line interval.Therefore, FIG. 1 shows that the written image is effectively correctedto have reduced distortion compared with the case where an image iswritten continuously along one scanning line, that is, the scanning lineSL3. In such a case, the written image curves along the scanning lineSL3 so as to vary over four scanning line intervals of FIG. 1.

If the maximum value of scanning line curving is 0.5 mm for an effectivescanning region width of 290 mm, the magnitude of variation of thewritten image shown in the lower part of FIG. 1 is at most a little over0.1 mm in each of the regions D1 through D6. Therefore, it is apparentthat scanning line curving is effectively reduced. On a scale of thismagnitude, the variation of the image does not pose any problem.

As previously described, according to the divided writing method, theeffective scanning region is divided into the regions Di (i=1, 2, . . .) in accordance with the scanning line curving characteristic. Further,in each optical scan, one of the image data for a plurality of imagelines which one is suitable for the optical scan is selected in each ofthe regions Di. Thereby, the image data for each image line is writtenwith the corrected scanning line curving.

FIGS. 2A and 2B are diagrams for illustrating an imaging apparatusaccording to a first embodiment of the present invention.

As shown in FIG. 2B, this imaging apparatus is a color-image formingapparatus of a tandem type. The imaging apparatus includes fourphotoconductive photosensitive bodies 8A′, 8A, 8B, and 8B′ arranged inthe order described along a conveyance path for conveying a sheet-likerecording medium (not shown in the drawings) from right to left in FIG.2B. The four photosensitive bodies 8A′, 8A, 8B, and 8B′ form substantialscanning surfaces, and are subjected to optical scanning individually.The sheet-like recording medium may be transfer paper or an OHP sheet (aplastic sheet for overhead projectors), for instance.

As shown in FIGS. 2A and 2B, the optical scanner of the imagingapparatus has superimposed optical systems in the axial direction of apolygon mirror 4 between light sources and scanning and imaging opticalsystems. The polygon mirror 4 is a rotary polygonal mirror serving as anoptical deflection and scanning part.

As shown in FIG. 2A, in the superimposed optical systems of the opticalscanner, a light beam emitted from a light source 1A, which is asemiconductor laser, is coupled by a coupling lens 2A to be convertedinto a parallel light beam or a weak divergent or convergent light beam.Then, the light beam is beam-shaped by passing through an aperture APfor beam shaping. Thereafter, the light beam is transmitted through acylindrical lens 3A that is a line-image forming optical system to becondensed in the sub scanning direction. Then, the light beam is focusedapproximately (or exactly) on a deflecting and reflecting surface of thepolygon mirror 4 as a line image extending along the main scanningdirection.

A light beam emitted from a light source 1A′ (semiconductor laser)superimposed on the above-described optical system is coupled by acoupling lens 2A′ to be converted into a parallel light beam or a weakdivergent or convergent light beam. Then, the light beam is beam-shapedby passing through an aperture AP′ for beam shaping. Thereafter, thelight beam is transmitted through a cylindrical lens 3A′ that is aline-image forming optical system to be condensed in the sub scanningdirection. Then, the light beam is focused approximately on a deflectingand reflecting surface of the polygon mirror 4 as a line image extendingalong the main scanning direction. The line image formed by the lightbeam emitted from the light source 1A′ is separated from that formed bythe light beam emitted from the light source 1A in the axial directionof the polygon mirror 4 or in the sub scanning direction.

Light sources 1B and 1B′ that are semiconductor lasers, coupling lenses2B and 2B′, apertures BP and BP′, and cylindrical lenses 3B and 3B′ arealso superimposed, respectively, in the sub scanning direction or in theaxial direction of the polygon mirror 4.

A light beam emitted from a light source 1B (1B′) is coupled by acoupling lens 2B (2B′) to be converted into a parallel light beam or aweak divergent or convergent light beam. Then, the light beam isbeam-shaped by passing through an aperture BP (BP′) for beam shaping.Thereafter, the light beam is transmitted through a cylindrical lens 3A(3A′) that is a line-image forming optical system to be condensed in thesub scanning direction. Then, the light beam is focused approximately ona deflecting and reflecting surface of the polygon mirror 4 as a lineimage extending along the main scanning direction. The line images intowhich the light beams emitted from the light source 1B and 1B′ areformed, respectively, are also separated from each other in the subscanning direction.

As shown in FIG. 2B, the polygon mirror 4 has deflecting and reflectingsurfaces wide in the direction of its rotation axis so as to be capableof simultaneously deflecting the four light beams for scanning.

The light beam emitted from the light source 1A is deflected by thepolygon mirror 4 to pass through a lens 5A. Then, the light beam isreflected by mirrors MA1, MA2, and MA3 for optical path bending, andpasses through a lens 6A to be guided to the photoconductivephotosensitive body 8A. The light beam is focused into a light spot(beam spot) on the photosensitive body 8A through the action of thelenses 5A and 6A forming an fθ lens, and optically scans thephotosensitive body 8A.

The light beam emitted from the light source 1A′ is deflected by thepolygon mirror 4 to pass through a lens 5A′. Then, the light beam isreflected by a mirror MA for optical path bending, and passes through alens 6A′ to be guided to the photoconductive photosensitive body 8A′.The light beam is focused into a light spot (beam spot) on thephotosensitive body 8A′ through the action of the lenses 5A′ and 6A′forming an fθ lens, and optically scans the photosensitive body 8A′.

The light beam emitted from the light source 1B is deflected by thepolygon mirror 4 to pass through a lens 5B. Then, the light beam isreflected by mirrors MB1, MB2, and MB3 for optical path bending, andpasses through a lens 6B to be guided onto the photoconductivephotosensitive body 8B. The light beam is focused into a light spot(beam spot) on the photosensitive body 8B through the action of thelenses 5B and 6B forming an fθ lens, and optically scans thephotosensitive body 8B.

The light beam emitted from the light source 1B′ is deflected by thepolygon mirror 4 to pass through a lens 5B′. Then, the light beam isreflected by a mirror MB for optical path bending, and passes through alens 6B′ to be guided to the photoconductive photosensitive body 8B′.The light beam is focused into a light spot (beam spot) on thephotosensitive body 8B′ through the action of the lenses 5B′ and 6B′forming an fθ lens, and optically scans the photosensitive body 8B′.

For instance, electrostatic latent images corresponding to yellow,magenta, cyan, and black images are written to and formed on thephotosensitive bodies 8A′, 8A, 8B, and 8B′, respectively. Then, thelatent images are made visible with their corresponding color toners soas to be formed into a yellow toner image, a magenta toner image, a cyantoner image, and a black toner image, respectively. These color imagesare transferred to the sheet-like recording medium, positioned relativeto one another and superimposed one over the other. After fixing thetransferred color images, a composite color image is obtained.

At this point, application of image data modulating the light emissionintensity of each of the light sources 1A, 1A′, 1B and 1B′ is controlledby a control part such as a computer (not shown in the drawings), andthe “divided writing method” is performed as described above withreference to FIG. 1. Thereby, scanning line curving is effectivelycorrected in each of the images written to the photosensitive bodies 8A,8A′, 8B, and 8B′. Therefore, the above-described problem of colormisregistration due to scanning line curving can be effectively reduced.

According to the optical scanner shown in FIGS. 2A and 2B to which thedivided writing method of FIG. 1 is applied, the light beams emittedfrom the light sources 1A, 1A′, 1B, and 1B′ are deflected in the mainscanning direction by the optical deflection and scanning part 4 to becondensed toward the scanning surfaces 8A, 8A′, 8B, and 8B′ by thescanning and imaging optical systems 5A and 6A, 5A′ and 6A′, 5B and 6B,and 5B′ and 6B′, respectively. Thereby, the light spots are formed onthe scanning surfaces 8A, 8A′, 8B, and 8B′, so that the optical scanneroptically scans the scanning surfaces 8A, 8A′ 8B, and 8B′ with the lightspots. The effective scanning region of each of the scanning surfaces8A, 8A′, 8B, and 8B′ is divided into the regions Di (i=1, 2, . . . ) inaccordance with the scanning line curving characteristic. In eachoptical scan, one of the image data for a plurality of image lines whichone is suitable for the optical scan is selected in each of the regionsDi. Thereby, the image data for each image line is written withcorrected scanning line curving.

According to the above-described optical scanner, scanning line curvingis effectively corrected as described above. This correction is notalways perfect, however, as shown in the written image for the imageline 3 shown in the lower part of FIG. 1. Therefore, further correctionfor fine adjustment is required.

Next, a description will be given of this correction for fine adjustmentaccording to the first embodiment of the present invention.

According to a method of correction for fine adjustment according to thefirst embodiment of the present invention, a plurality of light beamsare emitted from a light source so that two or more light spots areformed close to each other in the sub scanning direction on a scanningsurface. The “center of gravity (CG)” position in the composite lightintensity distribution obtained by superimposing the light intensitydistributions of the light beams in the sub scanning direction is variedor shifted by adjusting the intensity of each of the light beams.Thereby, scanning line curving that is included in one image line but isnot completely correctable by the divided writing method is finelyadjusted to be corrected.

FIGS. 3A through 3B are diagrams for illustrating the method ofcorrection for fine adjustment according to the first embodiment of thepresent invention.

In FIG. 3A, reference numerals B1 and B2 denote the traces of movementof two light spots b1 and b2 that are formed on a scanning surface bytwo light beams emitted from a light source. The traces of movement ofthe two light spots are close to each other in the sub scanningdirection.

The light intensity distributions of the light spots b1 and b2 on thescanning surface view from the main scanning direction are shown in (b)of FIG. 3B. The left-to-right direction of FIG. 3B corresponds to thesub scanning direction. As shown in (b) of FIG. 3B, when the lightintensity distributions of the light spots b1 and b2 are equal to eachother, the light spots b1 and b2 close to each other in the sub scanningdirection are combined into a composite light intensity distribution SP0indicated by the solid line in (b) of FIG. 3B.

The above-mentioned center of gravity position in a composite lightintensity distribution obtained by superimposing the light intensitydistributions of the light beams in the sub scanning directioncorresponds to a maximum intensity position in the sub scanningdirection in the composite light intensity distribution SP0 in the caseof (b) of FIG. 3B.

When the light intensity distributions of the light spots b1 and b2 areset to be equal to each other during optical scanning, the two lightspots b1 and b2 draw the traces of movement B1 and B2, respectively, sothat the composite light intensity distribution SP0 draws a trace SLindicated by the solid line in FIG. 3A. Since the scanning surface issubstantially scanned by the composite light intensity distribution SP0,a scanning line on the scanned surface matches the trace SL.

FIG. 3B(a) shows the case where the light intensities of the light spotsb1 and b2 are imbalanced so that the light intensity of the light spot 2is lower than that of the light spot b1. In this case, the center ofgravity position of a composite light intensity distribution SP1 shiftsto the left as shown in (a) of FIG. 3B. FIG. 3B(c) shows the case wherethe light intensities of the light spots b1 and b2 are imbalanced sothat the light intensity of the light spot b1 is lower than that of thelight spot b2. In this case, the center of gravity position of acomposite light intensity distribution SP2 shifts to the right as shownin (c) of FIG. 3B.

Accordingly, when optical scanning is performed with the composite lightintensity distribution SP1, the scanning line SL approaches the trace B1on the scanning surface in FIG. 3A. On the other hand, when opticalscanning is performed with the composite light intensity distributionSP2, the scanning line SL approaches the trace B2 on the scanningsurface in FIG. 3A.

That is, by adjusting the magnitudes of the light intensitydistributions of the light spots b1 and b2 relative to each other, thecenter of gravity position in the composite light intensity distribution(the position of the center of a composite light spot optically scanningthe scanning surface) can be shifted along the sub scanning direction.

For instance, when the scanning line SL is formed as shown in FIG. 3C ina certain scanning region Di as a result of correcting scanning linecurving by the above-described divided writing method, the lightintensity of the light spot b2 is set to be higher than that of thelight spot b1 in the section di1 of the region Di, the light intensityof the light spot b1 is set to be higher than that of the light spot b2in the section di2 of the region Di, and the light intensity of thelight spot b2 is set to be higher than that of the light spot b1 in thesection di3 of the region Di. Thereby, the center of gravity of thecomposite light intensity distribution approaches the image line, sothat the scanning line SL, which is the trace of the center of gravityof the composite light intensity distribution, can substantially matchthe image line very well as shown in FIG. 3D.

In the divided writing method, the scanning line can better match theimage line also in another divided region Dj. Thus, scanning linecurving that is not completely correctable by the divided writing methodis subjected to further correction for fine adjustment. Thereby, anexcellent scanning line can be realized.

In the above description, the light spots b1 and b2 are aligned in thesub scanning direction. Actually, however, the light spots b1 and b2 arenot necessarily required to be aligned completely in the sub scanningdirection. The light spots b1 and b2 may be offset relative to eachother in the main scanning direction. There may be a slight differencein timing between exposure by the light spot b1 and exposure by thelight spot b2 such that the difference does not exert great influence onthe composite exposure distribution. That is, exposure of aphotosensitive medium forming a substantial scanning surface is thescalar sum of the light intensities of the light spots b1 and b2, sothat the light spots b1 and b2 are not necessarily required to exposethe photosensitive body to light simultaneously. Further, the lightspots b1 and b2 can be formed with different timings by successiveoptical scans by the same light beam.

In the above-described case, the two light spots b1 and b2 are employed.The scanning line, however, can be formed of the composite lightintensity distribution of three or more light spots. In this case, byadjusting the light intensity distributions of the light spots relativeto each other, correction can be performed with finer adjustment.

In the case of thus performing correction for fine adjustment onscanning line curving by shifting the center of gravity of the compositelight intensity distributions of a plurality of light spots, in order toprevent the light spots from being separated from each other and thusdegrading resolution in the sub scanning direction, it is preferablethat the diameter Ws in the sub scanning direction of a stationary lightspot on the scanning surface and the distance L between the scanningtraces of adjacent light spots satisfy the following expression, Ws andL being defined by the maximum intensity 1/e² of the light intensitydistribution of each light spot:1.2<Ws/L<4.5

When the parameter Ws/L is smaller than or equal to the lower limitvalue 1.2, the overlap of the adjacent light spots is reduced, so thatthe composite light intensity distribution of the light spots isprevented from having a “single-peaked shape,” thus making it difficultto perform optical scanning by the composite light intensitydistribution. On the other hand, when the parameter Ws/L is larger thanor equal to the upper limit value 4.5, the composite light intensitydistribution is reduced in width in the sub scanning direction so as tonarrow the shift range of the center of gravity of the composite lightintensity distribution. This makes it difficult to perform effectivecorrection for fine adjustment.

In the case of performing this method of correction for fine adjustmentin the imaging apparatus shown in FIGS. 2A and 2B, each light sourceemploys, as a light emitting source, a monolithic semiconductor laserarray of two or more semiconductor laser light emitting parts arrangedin an array. The light beams emitted from the semiconductor laser lightemitting parts are formed into a plurality of light spots close to eachother in the sub scanning direction on the scanning surface, and theintensity of each of the light beams emitted from the semiconductorlaser light emitting parts is modulated.

There is another method of performing further correction for fineadjustment on scanning line curving corrected by the divided writingmethod.

According to this method, by using a deflection part that can deflect alight beam emitted from a light source in the sub scanning direction, alight spot position on the scanning surface is displaced in the subscanning direction by deflecting the light beam with the deflectionpart, thereby finely adjusting and correcting scanning line curvingincluded in one scanning line but not completely correctable by thedivided writing method.

As the deflection part, a liquid crystal deflection element or adeflection mirror may be employed. A description will first be given ofthe case of employing a liquid crystal deflection element.

The liquid crystal deflection element is an optical deflector thatperforms optical deflection by using a liquid crystal. A variety ofconventional liquid crystal deflection elements are known. The liquidcrystal deflection elements are categorized into two types: those drivenby an electrical signal and those driven by a magnetic signal. Here, adescription will be given of the former type of liquid crystaldeflection element.

The liquid crystal deflection elements driven by an electrical signalare roughly divided into two types. In the first type, the electricalsignal varies the refractive indexes of the liquid crystal deflectionelements. In the second type, the electrical signal causes diffractionin the liquid crystal deflection elements.

An example of the first type of liquid crystal deflection element usingvariation in the refractive index is disclosed in Japanese Laid-OpenPatent Application 8-313941. According to the liquid crystal deflectionelement disclosed in this patent application, a liquid crystal isprovided with a refractive index gradient so as to deflect a light beampassing through the liquid crystal. As an electrical signal driving thisliquid crystal deflection element, a modulated signal having apulse-like or sinusoidal waveform and having an average voltage ofapproximately zero volts is preferred. In this case, deflection can becontrolled by varying the pulse duty ratio of the pulse signal.

FIG. 4 is a diagram showing a liquid crystal deflection element of thesecond type, which causes diffraction by an electrical signal. This typeof liquid crystal deflection element is expatiated in theabove-described patent application (8-313941).

According to FIG. 4, a liquid crystal deflection element 17 is formed ofa liquid crystal cell 171 and a driving circuit 172 driving the liquidcrystal cell 171. The liquid crystal cell 171 includes a pair ofopposing glass substrates 1711 and 1712 parallel to each other.Transparent electrodes 1713 and 1714 are formed of ITO on the opposingsurfaces of the glass substrates 1711 and 1712, respectively. Alignmentfilms 1715 and 1716 are formed on the transparent electrodes 1713 and1714 so that a liquid crystal 170 is sealed in the form of a thin filmin a space defined by the alignment films 1715 and 1716 and spacers 1717and 1718 interposed therebetween. The driving circuit 172 applies adriving voltage between the transparent electrodes 1713 and 1714.

The liquid crystal 170 is, for instance, a nematic liquid crystal havingnegative dielectric anisotropy in which the dielectric constant issmaller in the axial direction of a liquid crystal molecule than in thedirection perpendicular to the axial direction. The molecules of theliquid crystal 170 are aligned by the alignment films 1715 and 1716 sothat the axial direction of the molecules is perpendicular to the X-Zplane of FIG. 4.

When a DC voltage or a voltage of a frequency lower than or equal toapproximately 300 Hz is applied between the transparent electrodes 1713and 1714 by the driving circuit 172, a diffraction grating pattern isformed in the liquid crystal 170 so that its stripe pattern extends in adirection parallel to the alignment direction of the molecules (that is,its stripe pattern extends along the Y-axis of FIG. 4).

In this state, when a light beam is made incident on the liquid crystalcell 171 as shown in FIG. 4, the diffraction grating pattern generates adiffracted light beam of the transmitted light. When the value of the DCor low-frequency voltage is varied, the diffraction grating pattern isformed with a different grating pitch so as to have a different angle ofdiffraction.

Accordingly, with respect to 1^(st)-order diffracted light, forinstance, by adjusting the deflection angle of the 1^(st)-orderdiffracted light, the light beam can be deflected at a desireddeflection angle in a desired direction (along the X-Z plane in the caseof FIG. 4). On the other hand, when a high-frequency voltage is appliedbetween the transparent electrodes 1713 and 1714 of the liquid crystalcell 171, the diffraction grating pattern is formed in the liquidcrystal 170 in the direction perpendicular to the alignment direction ofthe molecules, so that diffracted light in a direction along the X-Yplane is obtained. In this case, the angle of diffraction can be variedby changing the envelope of the applied high-frequency voltage.

FIGS. 5A and 5B are diagrams showing optical scanners performingcorrection for fine adjustment on scanning line curving using a liquidcrystal deflection element according to the first embodiment of thepresent invention.

According to the optical scanner of FIG. 5A, a parallel light beam or aweak divergent or convergent light beam is emitted from a light sourcedevice 10 formed of a semiconductor laser and a coupling lens to betransmitted through a liquid crystal deflection element 17. Then, thelight beam is condensed in the sub scanning direction by a cylindricallens 12 so as to be focused approximately (or exactly) on a deflectingand reflecting surface of a polygon mirror 14 as a line image extendingalong the main scanning direction.

The deflected light beam reflected from the polygon mirror 14 passesthrough lenses 161 and 162 forming an fθ lens 16 as a scanning andimaging optical system to be focused into a light spot on a scanningsurface 20, thereby performing optical scanning. At this point,recombination of image data is performed through the operation of acontrol part such as a microcomputer (not shown in the drawing) so thatan image is written to the scanning surface 20 by the divided writingmethod.

In the optical scanner of FIG. 5A, the liquid crystal deflection element17 and a controller 22 form a deflection part. The controller 22 is setas one of the functions of the control part controlling the entireimaging apparatus.

As described above with reference to FIG. 4, the liquid crystaldeflection element 17 deflects a light beam by using diffraction.However, the liquid crystal deflection element 17 may deflect a lightbeam by forming a refractive index distribution. Further, the liquidcrystal deflection element 17 may be driven by a magnetic signal.

Every time optical scanning is performed with the light spot, thecontroller 22 controls deflection of a light beam in the sub scanningdirection by the liquid crystal deflection element 17, therebyperforming correction for fine adjustment on scanning line curving thatis not completely correctable by the divided writing method.

The optical scanner of FIG. 5B includes a liquid crystal deflectionelement array 18 and the controller 22 that form a deflection part. Theliquid crystal deflection element array 18 is formed of a plurality ofliquid crystal deflection elements having the same function as theliquid crystal deflection element 17 of FIG. 5B, the liquid crystaldeflection elements being arranged in an array along the main scanningdirection. Each of the liquid crystal deflection elements forming thearray performs correction for fine adjustment in a correspondingpredetermined section. In the optical scanner of FIG. 5A, every time thelight spot performs optical scanning, the liquid crystal deflectionelement 17 is required to vary its deflection for correction for fineadjustment during the optical scanning. Meanwhile, the liquid crystaldeflection element array 18 of FIG. 5B enables correction for fineadjustment to be performed at a time in every optical scan. Therefore,the liquid crystal deflection element array 18 has the merit of enablingemployment of a liquid crystal deflection element having a slow responsespeed.

In the above description, a liquid crystal deflection element isemployed as the deflection part of the optical scanner. However, thedeflection part for fine adjustment of scanning line curving can berealized otherwise than by the liquid crystal deflection element. Adeflection mirror may be employed as the deflection part. The deflectionmirror is an optical device that can vary the orientation of itsreflecting surface at high speed within a relatively narrow range. Avariety of deflection mirrors are known. FIGS. 6A and 6B are diagramsshowing typical deflection mirrors.

The deflection mirror of FIG. 6A employs magnetic force as its drivingforce.

In the deflection mirror of FIG. 6A, a substrate 61 is supportedrotatably about an axis 62 by a support mechanism (not shown in thedrawing). A mirror face 67 is formed on one side of the substrate 61. Acoil 65 is formed so as to surround the mirror face 67. Thereby, avibration element is formed. Stationary magnets 63 and 64 are fixedlyprovided from the upper and lower sides of the vibration element so asto sandwich the vibration element therebetween, thereby providingmagnetic fields in the vertical directions of the vibration element. Aresilient force is exerted on the substrate 61 by a spring (not shown inthe drawing) so as to maintain the substrate 61 at a neutral position.

When a driving current is supplied from a driving circuit 66 to the coil65, the Lorentz force is generated by the interaction between thedriving current flowing through the coil 65 and the magnetic fieldsprovided by the stationary magnets 63 and 64. The Lorentz forcegenerates torque in the vibration element, thereby varying theorientation of the mirror face 67. The driving current supplied to thecoil 65 may be a rectangular wave, a sinusoidal wave, or a DC current.In the case of supplying a DC current to the coil 65, the mirror face 67can be maintained and fixed at a certain orientation.

By controlling the driving current supplied to the coil 65, theorientation of the mirror face 67 can be controlled. The substrate 61 isformed of silicon. The coil 65 and the mirror face 67 are formed in thinfilms by vapor deposition or plating. By thus forming the coil 65 andthe mirror face 67 on the single silicon substrate 61, a lightweightdeflection mirror can be manufactured at low cost by using semiconductorprocessing. Since the vibration element is lightweight, high-speeddriving can be realized at low energy.

The deflection mirror of FIG. 6B takes advantage of the piezoelectriceffect.

The deflection mirror of FIG. 6B includes a stationary plate 68 and arotatable substrate 69 that are engaged like a hinge so that therotatable substrate 69 can rotate about an axis 72. A mirror face 70 isformed in a thin film on the rotatable substrate 69 by vapor evaporationor plating. Instead of using vapor evaporation, the mirror face 70 maybe formed by forming the rotatable substrate 69 of metal aluminum andperforming mirror-finishing on the surface thereof.

A piezoelectric element 71 is fixed so as to be sandwiched between thefree end of the rotatable substrate 69 and the free end of thestationary plate 68. The piezoelectric element 71 is driven by a drivingcircuit 73 generating an electrical signal of a frequency higher than orequal to 2 KHz, thereby rotating the rotatable substrate 69 about theaxis 72.

By providing such a deflection mirror as is described with reference toFIG. 6A or 6B between a light source and an optical deflection andscanning part (polygon mirror) so as to deflect a light beam emittedfrom the light source in the sub scanning direction, scanning linecurving that is not completely correctable by the divided writing methodcan be finely adjusted to be corrected.

Each of FIGS. 6C and 6D shows a configuration where a deflection mirror75 such as any of the foregoing deflection mirrors is provided betweenthe cylindrical lens 12 and the optical deflection and scanning part soas to deflect a light beam emitted from the light source in the subscanning direction. For instance, by removing the liquid crystaldeflection element 17 and the liquid crystal deflection element array 18from the optical scanners of FIGS. 5A and 5B and changing the systembetween the light source device 10 and the polygon mirror 14 as shown inFIGS. 6C and 6D, respectively, correction for fine adjustment can berealized by using the deflection mirror 75.

According to the first embodiment of the present invention, correctionfor fine adjustment using the liquid crystal deflection element or thedeflection mirror as the deflection part can also be realized in theoptical scanner shown in FIGS. 2A and 2B.

In the above description, scanning line curving (including scanning linetilting), which is a deviation from an ideal scanning position in thesub scanning direction, is corrected. As previously described, as ascanning characteristic, there also exists a deviation of the scanningposition of a light spot in the main scanning direction, the deviationresulting from the constant velocity characteristic (fθ characteristic)as a scanning characteristic in the main scanning direction. Thedeviation of a scanning position in the main scanning direction and thedeviation of a scanning position in the sub scanning direction aresubstantially independent of each other. Therefore, the deviation in themain scanning direction and the deviation in the sub scanning directioncan be corrected simultaneously.

In addition to the above-described correction of scanning line curving,the optical scanner according to the first embodiment of the presentinvention may correctively adjust the position of a light spot in themain scanning direction. For this purpose, the optical scanner accordingto the first embodiment of the present invention shifts the phase of thepixel clock signal of image data to be written based on a clock signalwhose frequency is higher than that of the pixel clock signal.

The pixel clock signal is a clock signal for causing a light spot towrite one pixel.

To correctively adjust the position of a light spot in the main scanningdirection means to adjust the light spot position on the scanningsurface with respect to each image height so that the light spotposition matches or approximates the ideal position in the main scanningdirection.

Conventionally, electrical correction of a positional deviation of alight spot in the main scanning direction is known, the deviationresulting from the fθ characteristic. This deviation results from thefact that the light spot does not move in the main scanning direction ata constant speed. Basically, therefore, the dot position at which thelight spot writes a pixel can be adjusted, where the light spot moves ata speed higher (lower) than a predetermined speed (at which the ideal fθcharacteristic is realized), by advancing (delaying) the rise timing ofthe pixel clock signal and simultaneously reducing (increasing) the timewidth of the pixel clock signal.

By performing this adjustment pixel by pixel, a positional deviation ofthe light spot in the main scanning direction can be completelycorrected in principle.

FIG. 7A shows a configuration for adjusting the position of a light spotin the main scanning direction according to the first embodiment of thepresent invention.

As shown in FIG. 7A, photosensors 701 and 702 are provided on both sidesof the effective scanning region of the light spot formed on a scanningsurface 700 by the deflected light beam of the optical scanner. Whenoptical scanning is performed, a time counter 703 counts the time thatthe light spot takes to move between the photosensors 701 and 702, andthe count value of the time counter 703 is input to a look-up table 704.

The fθ characteristic (constant velocity characteristic) in the opticalscanner is known. The look-up table 704 stores positional corrections tobe made in the main scanning direction so that the positionalcorrections are correlated with their corresponding time values countedby the time counter 703. Thus, the correction (value) corresponding tothe count value of the time counter 703 is set based on the storedcontents of the look-up table 704.

The correction value is applied to a phase-locked loop (PLL) 705. Aclock signal is input from a clock generator circuit 706 to the PLL 705.The clock signal generated in the clock generator circuit 706 has afrequency higher than that of the pixel clock signal. When the pixelclock signal is generated based on the clock signal whose frequency is n(for instance, eight) times that of the pixel clock signal, with respectto the reference length T of the pixel clock signal, the rise timing(phase) of the pixel clock signal can be shifted in units of T/n, andthe length of the pixel clock signal can be varied in units of T/n.

The PLL 705, using the clock signal supplied from the clock generatorcircuit 706, generates a pixel clock signal whose rise timing is shiftedand whose signal length is changed based on the correction valueobtained from the look-up table 704. Then, the PLL 705 applies thegenerated pixel clock signal to an image processing circuit 707. Theimage processing circuit applies image data and the pixel clock signalto a semiconductor laser driving circuit 708.

The semiconductor laser driving circuit 708 switches ON and OFF asemiconductor laser 709 in predetermined timing based on the pixel clocksignal in accordance with the image data. The light spot whose positionis adjusted in the main scanning direction writes the image data basedon the switching timing and the ON-time (light-emitting) period of thesemiconductor laser 709.

For instance, the deviation of a light spot position in the mainscanning direction based on the constant velocity characteristic variesas shown in (a) of FIG. 7B. In FIG. 7B, the vertical axis representspositional deviation in the main scanning direction, and the horizontalaxis represents the image height of the light spot.

In this case, as one method of correcting a positional deviation in themain scanning direction, the range of the image height is divided intoeight regions as shown in (a) of FIG. 7B. Here, the range of the imageheight is divided into eight regions for convenience of description andgraphical representation. Practically, however, it is appropriate todivide the range into fifteen to thirty regions. Further, it is betterfor the range of the image height to be unequally divided, than to beequally divided, so that part of the range where the light spot moves ata higher speed is divided into more regions.

Each of the regions thus divided is provided with the correspondingphase shift and signal length of the pixel clock signal. In each dividedregion, optical scanning is performed based on the pixel clock signalwith the corresponding phase shift and signal length determined for theregion. Thereby, the deviation of the light spot in the main scanningdirection is effectively reduced in each region of the image heightrange as shown in (b) of FIG. 7B compared with the uncorrected deviationshown in (a) of FIG. 7B. The greater the number of regions into whichthe image height range is divided, the more effectively a deviation inthe main scanning direction can be reduced.

The above-described adjustment (correction) of scanning line curving(including scanning line tilting) and a position in the main scanningdirection may be performed by optimizing the contents of the correctionat the time of initial adjustment of the imaging apparatus including theoptical scanner. Practically, however, when the imaging apparatusoperates for a long period of time, an object of correction may varyover time. Further, in the case of employing a resin optical device, itsoptical properties may vary due to changes in environmental conditionssuch as temperature and humidity, thereby causing variation in theobject of correction.

In this point of view, it is desirable to timely detect an occurrence ofscanning line curving and/or a deviation of a scanning position in themain scanning direction and optimize correction based on the detectionresults.

In order to achieve such correction, the optical scanner according tothe first embodiment of the present invention may include a scanningposition detecting part detecting the scanning position of a light spotformed by a deflected light beam.

FIGS. 8A and 8B are diagrams for illustrating a scanning positiondetecting part 80 according to the first embodiment of the presentinvention.

In FIG. 8A, the scanning position detecting part 80 includes a pluralityof area sensors Pi (i=1, 2, . . . ) so that the light-receiving surfacesof the area sensors Pi are arranged on a stationary plate 81 in the mainscanning direction. The area sensors Pi are provided so that thelight-receiving surfaces thereof occupy positions optically equivalentto the scanning surface. The area sensors Pi are scanned by a deflectedlight beam or a detection light beam that is a separated part of thedeflected light beam. Two-dimensional CCDs or CMOS sensors are employedas the area sensors Pi.

For instance, prior to performing an imaging process by opticalscanning, a light source is caused to emit light so that an optical scanis performed over the area sensors Pi of the scanning position detectingpart 80. At this point, the light source emits light at an interval sothat the deflected light beam or the detection light beam of eachemitted light is made incident on the corresponding area sensor Pi,thereby successively scanning the area sensors Pi.

The scanning position detecting part 80 outputs the position of thelight spot in the sub scanning direction detected by each area sensor Pito an operation part, which may be formed as part of the functions of acontrol part controlling the entire imaging apparatus, for instance.

In FIG. 8B, a group of black circles arranged along a curve S1 indicatethe thus detected positions in the sub scanning direction. In FIG. 8B,the broken line indicates an image line, which is straight in the mainscanning direction.

The controller approximates the form of a scanning line as a polynomialby the method of least squares based on the detected light spotpositions (scanning positions) in the sub scanning direction. Thepolynomial is the detected scanning line curving, which is indicated bythe curve S1.

A positional deviation of a light spot in the main scanning directiondue to the fθ characteristic may be specified by detecting the scanningposition of the light spot in the main scanning direction with each areasensor Pi.

A description will be given herein of detection of a scanning positionin the main scanning direction by the scanning position detecting part80.

As previously described, the scanning position detecting part 80 has thearea sensors Pi arranged so that the light-receiving surfaces thereofoccupy positions optically equivalent to the scanning surface. Further,the area sensors Pi are scanned by a deflected light beam or a detectionlight beam that is separated from the deflected light beam. FIGS. 9A and9B show specific configurations for detecting a scanning position inthis case.

In FIG. 9A, the Y-axis corresponds to the main scanning direction, andthe Z-axis corresponds to the sub scanning direction. In the opticalscanner of FIG. 5B, a scanning position is detected by a method as shownin FIG. 9A. The liquid crystal deflection element array 18 is providedwith a slight inclination to the sub scanning direction in the opticalpath of an imaging light beam deflected in the main scanning direction.

As a result, part of the imaging light beam is reflected from thesurface of incidence of the liquid crystal deflection element array 18to become a detection light beam LS. The detection light beam LS forms alight spot on the light-receiving surface of the scanning positiondetecting part 80, the light-receiving surface being provided on asurface equivalent to the scanning surface 20 with the surface ofincidence of the liquid crystal deflection element array 18 serving as areflecting surface. Thereby, the scanning position is detected.

In FIG. 9B, the Y-axis corresponds to the main scanning direction, andthe Z-axis corresponds to the sub scanning direction. An imaging lightbeam deflected in the main scanning direction and transmitted throughthe liquid crystal deflection element array 18 is reflected from areflecting surface member 19 provided with an inclination to the subscanning direction in the optical path of the imaging light beam. Then,the reflected light beam forms a light spot on the light-receivingsurface of the scanning position detecting part 80, the light-receivingsurface being provided on a surface equivalent to the scanning surface20 with respect to the reflecting surface of the reflecting surfacemember 19. Thereby, the scanning position is detected.

The reflecting surface member 19 may be formed of transparent glass tobe constantly positioned as shown in FIG. 9B so as to reflect andseparate part of the imaging light beam toward the scanning positiondetecting part 80. Alternatively, the reflecting surface member 19 maybe provided so as to be capable of moving in or out of the optical pathof the imaging light beam. In this case, the reflecting surface member19 is positioned in the optical path as shown in FIG. 9B only whenscanning position detection is performed.

The configurations of FIGS. 9A and 9B are employed in the case ofdetecting a scanning position by directly detecting a light spot formedby a deflected light beam. However, the scanning position is not alwaysrequired to be detected by detecting a light spot. FIGS. 9C and 9D showsother configurations for scanning position detection according to thefirst embodiment of the present invention.

In FIG. 9C, the scanning position is the position of a light spot on thelight-sensitive surface of a photoconductive photosensitive body 25substantially forming the scanning surface. Therefore, a linear tonerimage LTI obtained by making visible a latent image obtained byperforming writing for one scanning line on the photosensitive body 25by an imaging process is the visible image of the written scanning line.

The toner image LTI is illuminated with a lamp 26 so as to be focusedthrough an imaging system 27 on an image sensor 28. The image sensor 28reads the focused image, thereby detecting its scanning line curvingbased on the reading results. In this case, the lamp 26, the imagingsystem 27, and the image sensor 28 form the scanning position detectingpart.

In FIG. 9D, the linear toner image LTI formed on the photosensitive body25 as in the case of FIG. 9C is transferred through a transfer part 30onto an intermediate transfer belt 29 serving as an intermediatetransfer medium. The transferred toner image LTI is illuminated with thelamp 26 so as to be focused through the imaging system 27 on the imagesensor 28. The image sensor 28 reads the focused image, therebydetecting its scanning line curving based on the reading results. Inthis case, the lamp 26, the imaging system 27, and the image sensor 28also form the scanning position detecting part.

After the scanning line curving is detected, the linear toner image LTIis removed from the photosensitive body 25 in the case of FIG. 9A andfrom the photosensitive body 25 and the intermediate transfer belt 29 inthe case of FIG. 9B by a cleaning part (not shown in the drawing).

In order to detect a scanning position in the main scanning direction todetect the constant velocity characteristic, a required number of linearimages each having an appropriate length in the sub scanning directionare written in the main scanning direction parallel to each other andare made visible. Ideally, the linear images (straight lines) extendingin the sub scanning directions are equally spaced in the main scanningdirection. The visible straight lines are detected on the photosensitivebody 25 or the intermediate transfer belt 29, and the intervals betweenadjacent straight lines are checked. Thereby, the constant velocitycharacteristic can be determined.

The scanning position is thus detected by the scanning positiondetecting part according to the first embodiment of the presentinvention. The scanning position can also be detected as follows withoutusing the scanning position detecting part. That is, in this case, thetoner image LTI formed as described above is transferred and fixed to atransfer paper sheet, so that the degree of scanning line curving(including scanning line tilting) and the constant velocitycharacteristic are measured based on the image obtained on the transferpaper sheet. Based on the measurement results, deflection in each liquidcrystal deflection element of the liquid crystal deflection elementarray is determined.

When the optical system is not affected by environmental changes or isconfigured so as to automatically correct the effect of environmentalchanges, the degree of scanning line curving and the constant velocitycharacteristic are measured in advance to be stored in the control partas data for correction. Correction may be performed as described abovebased on the stored data for correction.

Second Embodiment

A description will be given of a second embodiment of the presentinvention.

FIG. 10 is a diagram for illustrating an optical scanner according tothe second embodiment of the present invention. FIG. 10 is similar informat to FIG. 1. In FIG. 10, reference numerals S1 through S5 denotefive successive image lines as in FIG. 1. The difference between FIGS. 1and 10 lies in that the five scanning lines SL1 through SL5 of FIG. 1are drawn by as many optical scans successively performed by a singlelight spot while the four scanning lines SL1 through SL4 of FIG. 10 aresimultaneously drawn by as many light spots. That is, according to thesecond embodiment of the present invention, the scanning surface isscanned line-sequentially by multi-beam scanning with four light spots.The scanning lines SL1 through SL4 have substantially the same scanningline curving characteristic.

That is, according to the optical scanner of the second embodiment ofthe present invention, a light beam emitted from a light sourcemodulated based on image data is deflected in the main scanningdirection by an optical deflection and scanning part. The deflectedlight beam is condensed toward a scanning surface by a scanning andimaging optical system so as to form a light spot on the scanningsurface. The scanning surface is optically scanned by the light spot.Particularly, in the optical scanner according to the second embodimentof the present invention, the light source emits a plurality of lightbeams so that the light beams form respective light spots on thescanning surface, the light spots being separated from each other in thesub scanning direction. The light spots are formed to have substantiallythe same scanning line curving characteristic, and the scanning surfaceis optically scanned simultaneously with the light spots.

When the light spots are spaced in the sub scanning direction at aninterval (intervals) substantially equal to an interval (intervals) atwhich image lines are spaced, and the optical system has substantiallythe same magnification for each image height, the scanning lines may beconsidered as including substantially the same scanning line curving.

The effective scanning region is divided into a plurality of regions Di(i=1, 2, . . . ) (four regions in the case of FIG. 10) based on thescanning line curving characteristic. In each optical scan using thelight spots simultaneously, one of the light beams is selected in eachregion Di of each image line, the one being suitable for opticalscanning in the corresponding region Di. Thereby, the image data foreach image line is written with the corrected scanning line curving.This writing method may be referred to as a “light-selective writingmethod.”

The light-selective writing method is a method that performs theabove-described divided writing method with multiple light beams. Sincethe divided writing method employs one light spot, the light spot writesinformation to different image lines as the light spots draws a curvedscanning line in one optical scan. However, in the case of opticallyscanning a scanning surface simultaneously by a plurality of light spotsspaced in the sub scanning direction, letting the number of light spotsbe N, writing can be performed to N image lines simultaneously.Therefore, when the divided writing method is applied to the opticalscan of each light spot, the most suitable one of the light spots isselected for writing each image line in each scanning region (because ofthis characteristic, this method is referred to as the light-selectivewriting method), and the N image lines can be written simultaneously.

In the case of writing an image to the image line S4 of FIG. 10, forinstance, the image is written by selecting: the light beam related toscanning by the scanning line SL2 in the region D1; the light beamrelated to scanning by the scanning line SL3 in the region D2; the lightbeam related to scanning by the scanning line SL4 in the region D3; andthe light beam related to scanning by the scanning line SL3 in theregion D4.

Similarly, the light-selective writing method is performedsimultaneously with respect to the other image lines S1, S2, S3, and S5.Thus, the scanning line curving is effectively corrected whilebenefiting from the high speed characteristic of multi-beam opticalscanning, thereby realizing excellent optical scanning.

The above-described methods of correction for fine adjustment are alsoapplicable to the optical scanner of the second embodiment of thepresent invention. Further, the optical scanner of the second embodimentcan also perform the above-described adjustment of a scanning positionin the main scanning direction in parallel with the correction ofscanning line curving.

Third Embodiment

FIG. 11 is a diagram showing an imaging apparatus according to a thirdembodiment of the present invention.

The imaging apparatus of FIG. 11 forms a color image by employingphotoconductive photosensitive bodies as photosensitive media. Accordingto the imaging apparatus, component images of four colors of magenta,cyan, yellow, and black are formed to be superimposed on a sheet-likerecording medium. Thereby, the color image is obtained.

The imaging apparatus includes polygon mirrors 151 and 152 having thesame shape. The polygon mirrors 151 and 152 are fixed to a common shaftso as to rotate together with the shaft as a single unit. The polygonmirrors 151 and 152, together with a driving part (not shown in thedrawing), form an optical deflection and scanning part.

The imaging apparatus further includes four light source devices, whichare not graphically represented in FIG. 11. Light beams emitted from twoof the light source devices are incident on the polygon mirror 151, andlight beams emitted from the other two of the light source devices areincident on the polygon mirror 152. The optical arrangement (system) ineach of the optical paths between the light source devices and thepolygon mirrors 151 and 152 of the imaging apparatus of FIG. 11 is equalto that of the imaging apparatus of the first embodiment shown in FIGS.2A and 2B.

Light beams LSY and LSK deflected by the polygon mirror 152 are lightbeams for writing the yellow component image and the black componentimage, respectively.

The intensity of the deflected light beam LSY is modulated byinformation for the yellow component image to pass through lenses LNY1and LNY2 forming an fθ lens as a scanning and imaging optical system.The lenses LNY1 and LNY2 are held on a holding body PTY. Then, thedeflected light beam LSY is reflected by optical path bending mirrorsMY1, MY2, and MY3 successively. Thus, the deflected light beam LSY isguided to the light-sensitive surface of a photoconductivephotosensitive body 150Y so as to optically scan the light-sensitivesurface, which substantially forms the scanning surface.

The photosensitive body 150Y is cylindrical. Being evenly charged by acharger CY while rotating in the direction of the arrow in FIG. 11, thephotosensitive body 150Y is optically scanned by the light spot of thedeflected light beam LSY. Thereby, the yellow component image is writtento the photosensitive body 150Y, so that a yellow latent image is formedthereon.

The intensity of the deflected light beam LSK is modulated byinformation for the black component image to pass through lenses LNK1and LNK2 forming an fθ lens. The lenses LNK1 and LNK2 are held on aholding body PTK. Then, the deflected light beam LSK is reflected byoptical path bending mirrors MK1, MK2, and MK3 successively. Thus, thedeflected light beam LSK is guided to the light-sensitive surface of aphotoconductive photosensitive body 150K so as to optically scan thelight-sensitive surface.

The photosensitive body 150K is cylindrical. Being evenly charged by acharger CK while rotating in the direction of the arrow in FIG. 11, thephotosensitive body 150K is optically scanned by the light spot of thedeflected light beam LSK. Thereby, the black component image is writtento the photosensitive body 150K, so that a black latent image is formedthereon.

Light beams LSM and LSC deflected by the polygon mirror 151 are lightbeams for writing the magenta component image and the cyan componentimage, respectively.

The intensity of the deflected light beam LSM is modulated byinformation for the magenta component image to pass through lenses LNM1and LNM2 forming an fθ lens. The lenses LNM1 and LNM2 are held on aholding body PTM. Then, the deflected light beam LSM is reflected byoptical path bending mirrors MM1, MM2, and MM3 successively. Thus, thedeflected light beam LSM is guided to the light-sensitive surface of aphotoconductive photosensitive body 150M so as to optically scan thelight-sensitive surface.

The photosensitive body 150M is cylindrical. Being evenly charged by acharger CM while rotating in the direction of the arrow in FIG. 11, thephotosensitive body 150M is optically scanned by the light spot of thedeflected light beam LSM. Thereby, the magenta component image iswritten to the photosensitive body 150M, so that a magenta latent imageis formed thereon.

The intensity of the deflected light beam LSC is modulated byinformation for the cyan component image to pass through lenses LNC1 andLNC2 forming an fθ lens. The lenses LNC1 and LNC2 are held on a holdingbody PTC. Then, the deflected light beam LSC is reflected by opticalpath bending mirrors MC1, MC2, and MC3 successively. Thus, the deflectedlight beam LSC is guided to the light-sensitive surface of aphotoconductive photosensitive body 150C so as to optically scan thelight-sensitive surface.

The photosensitive body 150C is cylindrical. Being evenly charged by acharger CC while rotating in the direction of the arrow in FIG. 11, thephotosensitive body 150C is optically scanned by the light spot of thedeflected light beam LSC. Thereby, the cyan component image is writtento the photosensitive body 150C, so that a cyan latent image is formedthereon.

Each of the photosensitive bodies 150Y, 150K, 150M, and 150C isoptically scanned by single-beam scanning in this embodiment. However,the photosensitive bodies 150Y, 150K, 150M, and 150C may be opticallyscanned by multi-beam scanning. In the case of employing single-beamscanning, the divided writing method described in the first embodimentis performed. In the case of employing multi-beam scanning, thelight-selective writing method described in the second embodiment isperformed. Further, the above-described adjustment of a scanningposition in the main scanning direction and/or the above-describedmethods of correction for fine adjustment are performed as required.

Recombination of image data required for the divided writing method orthe light-selective writing method is controlled by a control part (notshown in the drawing). The control part includes a microcomputer andcontrols the entire imaging apparatus.

Further, each of the chargers CY, CK, CM, and CC is of acorona-discharge type. However, a charger of a contact type, such as acharging roller or a charging brush, may be employed as each of thechargers CY, CK, CM, and CC.

The yellow, magenta, cyan, and black latent images formed on thephotosensitive bodies 150Y, 150M, 150C, and 150K, respectively, aredeveloped by corresponding development devices 153Y, 153M, 153C, and153K with the corresponding color toners (yellow, magenta, cyan, andblack toners) to be made visible.

Thus, the yellow toner image, the magenta toner image, the cyan tonerimage, and the black toner image are formed on the photosensitive bodies150Y, 150M, 150C, and 150K, respectively. These toner images aretransferred onto a transfer paper sheet P that is a sheet-like recordingmedium in the following manner.

As shown in FIG. 11, an endless conveyer belt 154 is fastened overpulleys 155 and 156 so as to contact the bottom of each of thephotosensitive bodies 150Y, 150M, 150C, and 150K. Inside the endlessconveyer belt 154, transfer devices 157Y, 157M, 157C, and 157K areprovided so as to oppose the corresponding photosensitive bodies 150Y,150M, 150C, and 150K, respectively, through the internal surface of theconveyer belt 154. In this embodiment, the transfer devices 157Y, 157M,157C, and 157K are of a corona-discharge type. However, a transferdevice of a contact type, such as a transfer roller, may be used as eachof the transfer devices 157Y, 157M, 157C, and 157K.

The transfer paper sheet P as a sheet-like recording medium is fed froma cassette 158 containing a stack of transfer paper sheets, and isplaced onto the conveyer belt 154 by feed rollers 159. Then, thetransfer paper sheet P is charged by a charger 160 to be attached to andheld on the outside surface of the conveyer belt 154 by electrostaticsuction. The conveyer belt 154 rotates counterclockwise to convey thetransfer paper sheet P held on its outside surface.

While the transfer paper sheet P is being conveyed as described above,first, the transfer device 157Y transfers the yellow toner image on thephotosensitive body 150Y to the transfer paper sheet P. Then, thetransfer devices 150M, 150C, and 150K successively transfer the magenta,cyan, and black toner images to the transfer paper sheet P. When thetoner images are transferred, the toner images are positioned relativeto one another so as to be superimposed one over the other.

Thus, the color image is formed on the transfer paper sheet P. Thetransfer paper sheet P with the color image is discharged by adischarger 161, and peels off the conveyer belt 154 due to its ownfirmness. Then, the color image is fixed on the transfer paper sheet Pby a fixing device 162, and the transfer paper sheet P is ejected byejection rollers 163 onto a tray 164, which also serves as the top plateof the imaging apparatus.

After the toner images are transferred, residual toner and paper powderare removed from the photosensitive bodies 150Y, 150M, 150C, and 150K bycorresponding cleaners 165Y, 165M, 165C, and 165K. Further, the conveyerbelt 154 is discharged by a discharger 166 and cleaned by a cleaner 167.

This is the outline of the imaging process in a full-color mode. Amonochrome black and white image can be formed by forming an image onlyon the photosensitive body 150K. The color image may be formed in two ormore (multiple) colors depending on the combinations of photosensitivebodies on which images are to be formed.

As a method of transferring color toner images to a transfer papersheet, the well-known transfer method that forms a color image on anintermediate transfer belt by transferring color toner images theretoand transfers the color image to a transfer paper sheet may be employedinstead of the method employed in this embodiment.

As previously described, in this color-image forming apparatus, itsscanning and imaging optical system is the four fθ lenses provided forthe deflected light beams LSY, LSM, LSC, and LSK, respectively. Each fθlens is formed of two lenses. These four fθ lenses are opticallyequivalent to one another. The optical paths from the light sourcedevices to the corresponding photosensitive bodies 150Y, 150M, 150C, and150K are designed to have the same length. These optical systemcomponents are provided in an optical housing 175 as shown in FIG. 11.

The lenses LNY1, LNM1, LNC1, and LNK1 are formed of the same resinmaterial. The lenses LNY2, LNM2, LNC2, and LNK2 are formed of the sameresin material. Polycarbonate, which is excellent in low waterabsorbency, high transparency, and formability, or a synthetic resinincluding polycarbonate as its main component is a suitable resinmaterial for these lenses. Using a resin material makes it easy to forman aspheric surface and also reduces material cost, therefore beingadvantageous in reducing the cost of the color-image forming apparatus.

On the other hand, the optical properties of a resin lens vary due tothe effect of changes in temperature and humidity. Therefore, the degreeof scanning line curving and tilting and the constant velocitycharacteristic also vary in accordance with environmental changes.Accordingly, liquid crystal deflection element arrays 170Y, 170M, 170C,and 170K, which are equal to the liquid crystal deflection element array18 shown in FIG. 5B, are provided in the optical paths of the deflectedlight beams LSY, LSM, LSC, and LSK scanning the photosensitive bodies150Y, 150M, 150C, and 150K, respectively, as shown in FIG. 11. Thereby,as previously described, the positions of the light spots on thephotosensitive bodies 150Y, 150M, 150C, and 150K are adjusted in the subscanning direction, and scanning line curving and the constant velocitycharacteristic are corrected by adjusting the positions of the lightspots in the main scanning direction by shifting the phases of imageclock signals.

In a tandem color-image forming apparatus as shown in FIG. 11, if thedegree of scanning line curving differs among images formed on thephotosensitive bodies, the problem of “color misregistration” becomesapparent. Therefore, by correcting scanning line curving with respect toeach of the photosensitive bodies as described above so that the imagesformed on the photosensitive bodies have substantially the same degreeof scanning line curving, the problem of “color misregistration” can beeffectively reduced or prevented.

Although not shown in FIG. 11, the scanning positions of the light spotsthat the deflected light beams LSY, LSM, LSC, and LSK form on thecorresponding photosensitive bodies 150Y, 150M, 150C, and 150K aredetected by the corresponding scanning position detecting parts (notshown in the drawing) equal to the scanning position detecting part 80shown in FIG. 8A. The scanning position detecting parts are provided atthe positions optically equivalent to the corresponding scanningsurfaces. In order to guide parts of the deflected light beams LSY, LSM,LSC, and LSK to the corresponding scanning position detecting parts, theliquid crystal deflection element arrays 170Y, 170M, 170C, and 170K areprovided with a slight inclination to the sub scanning direction in theoptical paths of the deflected light beams LSY, LSM, LSC, and LSK,respectively, thereby reflecting the detection light beams toward thecorresponding scanning position detecting parts as previously describedwith reference to FIG. 9A.

Instead of forming the entire scanning and imaging optical system of theimaging apparatus of resin lenses as described above, only one of thepairs of lenses, for instance, the paired lenses LNK1 and LNK2, may beformed, as an optical system serving as a scanning position reference,of a glass having a small coefficient of thermal expansion in order toeliminate the effect of temperature change. The scanning line curvingincluded in each of optical scans performed by the deflected light beamsLSY, LSM, and LSC and the scanning position in the main scanningdirection of each of the light spots formed by the deflected light beamsLSY, LSM, and LSC may be corrected by the corresponding one of theliquid crystal deflection element arrays 170Y, 170M, and 170C providedin the optical paths of the deflected light beams LSY, LSM, and LSC soas to match the scanning line curving and the light spot position in themain scanning direction of the deflected light LSK formed into an imageby the lenses LNK1 and LNK2.

In this case, the fθ lens formed by the lenses LNK1 and LNK2 isconfigured so as to be equivalent in optical properties to the other fθlenses. Further, the liquid crystal deflection element arrays 170Y,170M, and 170C are employed in the optical paths of the deflected lightbeams LSY, LSM, and LSC while no liquid crystal deflection element arrayis required in the optical path of the deflected light beam LSK, therebycausing a difference in length between the optical path of the deflectedlight beam LSK and the optical path of each of the deflected light beamsLSY, LSM, and LSC. Therefore, a transparent parallel plate equivalent inoptical thickness (physical thickness multiplied by refractive index) tothe liquid crystal deflection element array 170K is provided as itsreplacement in the optical path of the deflected light beam LSK, therebyeliminating the above-described difference in optical path length.

Thereby, the imaging apparatus is not required to provide liquid crystaldeflection element arrays in the optical paths of all the deflectedlight beams LSY, LSM, LSC, and LSK. Further, expensive glass lenses areused only for the reference scanning and imaging optical system (lensesLNK1 and LNK2), and the other scanning and imaging optical systems canbe formed of inexpensive plastic lenses. Therefore, the color-imageforming apparatus can be realized at low cost as a whole, and ahigh-quality color image with reduced color misregistration can beobtained.

That is, according to such a tandem color-image forming apparatus, N(≧2; four in this embodiment) photoconductive photosensitive bodies areprovided along the conveying path of a sheet-like recording medium.Different electrostatic latent images are written to and formed on thephotosensitive bodies by optical scanning. The electrostatic latentimages are made visible as toner images of different colors. The tonerimages are transferred to the sheet-like recording medium and fixed,being superimposed one over the other. An optical scanner is providedfor each of the photosensitive bodies. A black toner image is formed onone of the photosensitive bodies (photosensitive body 150K). The opticalscanners for the photosensitive bodies other than the one for the blacktoner image (photosensitive bodies 150Y, 150M, and 150C) may be any ofthe optical scanners of the above-described first and secondembodiments. Of scanning line curving and a light spot position in themain scanning direction, at least scanning line curving is correctedbased on the black toner image.

The electrostatic latent images are formed by evenly charging each ofthe photosensitive bodies and optically scanning the photosensitivebodies with the optical scanners.

According to the imaging apparatus of this embodiment, based on theblack toner image, scanning line curving in the toner images of theother colors is corrected. It is the black toner image that dominantlyaffects resolution (sharpness) and graininess (roughness) among theimage qualities of a color image. By correcting scanning line curving inthe toner images of the other colors based on the black toner image, ahigh-quality color image simultaneously satisfying resolution andgraininess as well as color misregistration can be obtained. Further,the amount of data requiring correction is reduced, thereby giving acost advantage.

The correction of scanning line curving according to the presentinvention may be effectively used in combination with the conventionalmechanical correction of scanning line curving.

The imaging apparatus of the third embodiment may be realized as anoptical color printer, a digital color copier, or a color facsimilemachine.

Fourth Embodiment

A description will now be given of a fourth embodiment of the presentinvention.

An optical scanner according to the fourth embodiment may have aconfiguration as shown in FIGS. 2A and 2B. Therefore, the abovedescription given with reference to FIGS. 2A and 2B is referred to forthe configuration and operation of the optical scanner of thisembodiment.

According to FIGS. 2A and 2B, the optical scanner of this embodimentincludes a plurality (four) optical scanning parts. In each of theoptical scanning parts, a light beam emitted from the modulated lightsource 1A (1A′, 1B, 1B′) is deflected by the optical deflection andscanning part 4. The deflected light beam is guided by the scanning andimaging optical system (lenses) 5A (5A′, 5B, 5B′) and 6A (6A′, 6B, 6B′)so as to form a light spot on the scanning surface 8A (8A′, 8B, 8B′).The scanning surface 8A (8A′, 8B, 8B′) is scanned by the optical spot.Images for composing a desired image are written separately to thecorresponding scanning surfaces (regions) 8A, 8A′, 8B, 8B′.

The light beam emitted from the modulated light source has the samemeaning as the above-described light beam emitted from the light sourcemodulated based on the image data.

The scanning region is substantially the photosensitive surface of aphotosensitive medium, and includes the above-described concept of thescanning surface. However, individual scanning regions may be separatescanning surfaces (the photosensitive surfaces of differentphotosensitive bodies, for instance), or a scanning surface may bedivided into different scanning regions.

That is, according to the optical scanner of this embodiment, eachoptical scanning part includes a combination of optical elements whichcombination guides a light beam emitted from the light source (1A, 1A′,1B, 1B′) and forms the light beam into a light spot on the correspondingscanning surface (8A, 8A′, 8B, 8B′). This combination is formed of, forinstance, the light source 1A, the coupling lens 2A, the aperture AP,the cylindrical lens 3A, the optical deflection and scanning part 4, thelenses 5A and 6A, and the mirrors MA1 through MA3 for optical pathbending. The light-sensitive surface of each of the photoconductivephotosensitive bodies 8A, 8A′, 8B, and 8B′ is an individual scanningsurface and at the same time the “scanning region (region to bescanned).” That is, the four scanning regions are arranged in the subscanning direction.

As previously described, the electrostatic latent images correspondingto the yellow, magenta, cyan, and black images are formed on thephotosensitive bodies 8A′, 8A, 8B, and 8B′, respectively. The latentimages are made visible with the corresponding color toners into theyellow, magenta, cyan, and black toner images, respectively. These tonerimages are transferred to a sheet-like recording medium (not shown inthe drawing), positioned relative to one another and superimposed oneover the other. The transferred toner images are fixed so that a colorimage is formed.

At this point, in at least one of the scanning regions, an exposuredistribution may be formed to have a center of gravity between adjacentscanning lines in the sub scanning direction so that the position of thecenter of gravity may be shifted in the sub scanning direction.According to this embodiment, the exposure distribution is formed tohave a center of gravity between adjacent scanning lines in the subscanning direction on each of the photosensitive bodies 8A′, 8A, and 8Bwhen the electrostatic latent images made visible by the color tonersother than the black toner are formed on the scanning region of each ofthe photosensitive bodies 8A′, 8A, and 8B.

The exposure distribution having the center of gravity between thescanning lines adjacent in the sub scanning direction refers to acomposite light intensity distribution formed by superimposing, in thesub scanning direction, the light intensity distributions of two lightspots formed adjacently in the sub scanning directions on scanning linesadjacent in the sub scanning direction on a scanning surface, thecomposite light intensity distribution having a center of gravitybetween the adjacent scanning lines. By varying the light intensities ofthe two light spots relative to each other so that the light intensityof one of the light spots becomes higher while the light intensity ofthe other becomes lower, the position of the center of gravity may beshifted in the sub scanning direction.

According to the optical scanner of the first embodiment, scanning linecurving that is included in one scanning line and not completelycorrectable by the divided writing method is corrected by adjusting theposition of the center of gravity of the thus formed exposuredistribution. On the other hand, according to the optical scanner of thefourth embodiment, the adjustment of the position of the center ofgravity is performed over the entire region of the scanning lines formedin the scanning region.

According to this embodiment, optical scanning may be performed on eachof the photosensitive bodies 8A′, 8A, and 8B by single-beam scanning sothat the exposure distribution is formed to have a center of gravitybetween adjacent scanning lines in the sub scanning direction bysuccessive optical scans.

A description will now be given of the case where the scanning lines onthe photosensitive bodies (scanning regions) 8A′, 8A, and 8B (referredto as a yellow, magenta, and cyan scanning line, respectively) arerelatively corrected based on the scanning line (referred to as a blackscanning line) on the photosensitive body 8B′ on which the electrostaticlatent image to be made visible with the black toner, and the colortoner images are superimposed one over the other so that the yellow,magenta, and cyan scanning lines are superimposed on the black scanningline.

Generally, the black scanning line includes a curve. In this embodiment,however, for convenience of description, the black scanning line is setto be straight and the other color scanning lines are linearlycorrected.

Since the yellow, magenta, and cyan scanning lines are corrected in thesame manner in principle, the case of correcting the yellow scanningline will be described.

FIGS. 12A through 12E are diagrams for illustrating the correction ofscanning line curving by the optical scanner according to the fourthembodiment of the present invention.

In FIG. 12A, reference numerals SL1 and SL2 refer to the scanning linesformed on the photosensitive body 8A′ by optical scanning. In FIG. 12A,the vertical directions correspond to the sub scanning direction. Aspreviously described, optical scanning is performed by single-beamscanning. Therefore, the scanning line SL1 is formed by one opticalscan, and the scanning line SL2 is formed by the subsequent opticalscan.

If the image written by each scanning line is a line extending in themain scanning direction, a light spot SP1 closely illuminates thescanning line SL1 so as to form a long linear latent image. Further, thescanning line SL2 is exposed by a light spot SP2.

FIG. 12A shows the ideal state where no scanning line curving exists inany of the scanning lines SL1 and SL2 (or the state where each of thescanning lines SL1 and SL2 matches its corresponding image line). Thescanning line SL1, which is straight in FIG. 12A, corresponds to thetrace of movement of the center of the light spot SP1. The light spotSP1 has a Gaussian light intensity distribution, and the center of thelight spot SP1 corresponds to the position of the part of maximumintensity of the light intensity distribution. Likewise, the straightscanning line SL2 corresponds to the trace of movement of the center ofthe light spot SP2.

Generally, scanning line curving exists in an actual scanning line. FIG.12B illustrates the case where the scanning line includes a curve, thatis, scanning line curving exists in the scanning line. As in theabove-described case of FIG. 12A, the scanning lines SL1 and SL2 in FIG.12B are the traces of movement of the light spots SP1 and SP2 of the twosuccessive optical scans. When the scanning line SL1 thus includes acurve, the composite exposure distribution of the light intensitydistribution of the light spot SP1 for optical scanning along thescanning line SL1 and the light intensity distribution of the light spotSP2 for optical scanning along the scanning line SL2 is used to correctthe scanning line SL1 and obtain a straight scanning line SL0.

In the position A in FIG. 12B, for instance, the straight scanning lineSL0, to which the scanning line SL1 is to be corrected, passes a point alittle closer to the scanning line SL1 between the scanning lines SL1and SL2.

Therefore, when the light spot SP1 performs optical scanning along thescanning line SL1, at the position A, the light spot SP1 performsexposure with a light intensity distribution SP1(A), which is “slightlystrong” as its light intensity distribution, as shown in (a) of FIG.12C. When the light spot SP2 successively performs optical scanningalong the scanning line SL2, at the position A, the light spot SP2performs exposure with a light intensity distribution SP2 (A), which is“slightly weak” as its light intensity distribution, as shown in (b) ofFIG. 12C. In FIG. 12C, the horizontal axis represents the sub scanningdirection, and the vertical directions represent exposure intensity.

Thereby, an exposure distribution SP(A), which is the sum of the lightintensity distribution SP1(A) and the light intensity distributionSP2(A), is formed at the position A as shown in (c) of FIG. 12C whenoptical scanning is performed along the scanning lines SL1 and SL2. Atthis point, the center of gravity position of the exposure distributionSP(A) becomes the position of the scanning line SL0 at the position A.That is, in (c) of FIG. 12C, the distance DA between the scanning lineSL1 and the center of gravity of the exposure distribution SP(A) at theposition A is equal to the difference between the scanning lines SL1 andSL0 at the position A shown in FIG. 12B.

As a method of adjusting the light intensity distribution of each lightspot in order to change the center of gravity position of the exposuredistribution, a method that changes modulation time for writing eachpixel (pulse width modulation), a method that changes the amount oflight of a light beam emitted from a light source (power modulation), orthe combination of these methods is employable.

At the position B in FIG. 12B, the scanning line SL0 is positionedsubstantially in the center between the scanning lines SL1 and SL2.Therefore, at this point, at the position B, the light spot SP1 performsexposure with a light intensity distribution SP1(B) as its lightintensity distribution and the light spot SP2 performs exposure with alight intensity distribution SP2 (B), which is substantially equal tothe light intensity distribution SP1(B), as its light intensitydistribution so that the resulting composite exposure distribution SP(B)has its center of gravity positioned substantially in the center betweenthe scanning lines SL1 and SL2 as shown in FIG. 12D.

At this point, the distance DB between the scanning line SL1 and thecenter of gravity of the exposure distribution SP(B) is equal to thedistance between the scanning lines SL1 and SL0 at the position B asshown in FIG. 12B.

At the position C in FIG. 12B, the scanning line SL0 is positionedcloser to the scanning line SL1 between the scanning lines SL1 and SL2.Therefore, at this point, at the position C, the light spot SP1 performsexposure with an increased light intensity distribution SP1(C) as shownin FIG. 12E and the light spot SP2 performs exposure with a reducedlight intensity distribution SP2(C) as shown in FIG. 12E so that theresulting composite exposure distribution SP(C) has its center ofgravity positioned closer to the scanning line SL1.

At this point, the distance DC between the scanning line SL1 and thecenter of gravity of the exposure distribution SP(C) is equal to thedistance between the scanning lines SL1 and SL0 at the position C asshown in FIG. 12B.

The light intensity distribution of the light spot SP1 for opticalscanning along the scanning line SL1 and the light intensitydistribution of the light spot SP2 for optical scanning along thescanning line SL2 are controlled relative to each other as describedabove over the entire optical scanning region so that the center ofgravity position of the exposure distribution formed by the light spotsSP1 and SP2 moves along the scanning line SL0. Thereby, the scanningline SL in which scanning line curving is linearly corrected can beobtained.

That is, when a dot line extending in the main scanning direction isformed on the scanning region (photosensitive body 8A′) on which anexposure distribution is formed between the scanning lines SL1 and SL2adjacent to each other in the sub scanning direction, the distancebetween one of the adjacent scanning lines (scanning line SL1) and thecenter of gravity of the exposure distribution in the sub scanningdirection may be varied according to the position in the main scanningdirection. In this case, the position of the center of gravity of theexposure distribution may be shifted in a stepped manner in the subscanning direction. Further, the distance between the one of theadjacent scanning lines (scanning line SL1) and the center of gravity ofthe exposure distribution in the sub scanning direction may be variedcontinuously according to the position in the main scanning direction.

As is apparent from the above description, the center of gravityposition of the exposure distribution formed by the light intensitydistributions of the light spots SP1 and SP2 formed by successiveoptical scans can be changed to a desired position between the two lightspots SP1 and SP2 by adjusting the sizes of the light intensitydistributions of the light spots SP1 and SP2 relative to each other.Therefore, not only the center of gravity position of the exposuredistribution can be adjusted to a desired straight line or curve, butalso the trace of the center of gravity position of the exposuredistribution (or the corrected scanning line) can be shifted in the subscanning direction.

Thus, the scanning line curving and the position in the sub scanningdirection of the yellow scanning line are adjusted so that the yellowscanning line can match the black scanning line. Thereby, the yellowtoner image can be superimposed suitably on the black toner image.

For instance, when a yellow scanning line SLY1 is inclined to a blackscanning line SLBL as shown in FIG. 13, the center of gravity positionSLY of the exposure distribution formed by a light spot for opticalscanning along the yellow scanning line SLY1 and a light spot foroptical scanning along a yellow scanning line SLY2 can be matched withthe black scanning line SLBL. That is, in this case, the intensity ofthe light spot for optical scanning along the yellow scanning line SLY1is controlled so as to increase as the optical scanning position movesfrom left to right in FIG. 13, and the intensity of the light spot foroptical scanning along the yellow scanning line SLY2 is controlled so asto be high at the beginning and gradually decrease thereafter.

The above-described case is an example of forming a corrected scanningline between the scanning lines SL1 and SL2 with the center of gravityof an exposure distribution by correcting the scanning lines SL1 andSL2. However, such correction may not be enough to straighten thecorrected scanning line if the scanning line includes a curve of acertain size. In such a case, the scanning line can be corrected asdesired by employing light spots for optical scanning along three ormore scanning lines. FIGS. 14A through 14E show such a case.

In FIG. 14A, the scanning lines SL1, SL2, and SL3 are the traces oflight spots in three successive optical scans. In the case of obtainingthe substantially straight scanning line SL0 by correcting scanning linecurving, for instance, at the position A, exposure is performed with thelight intensity distribution SP3(A) of the light spot for opticalscanning along the scanning line SL3 as shown in FIG. 14B since theposition of the scanning line SL0 matches the position of the scanningline SL3 at the position A.

At the position B, exposure is performed with the light intensitydistribution SP2(B) of the light spot for optical scanning along thescanning line SL2 as shown in FIG. 14C. At the position C, the center ofgravity position of the composite exposure distribution of the lightintensity distribution SP1(C) of the light spot for optical scanningalong the scanning line SL1 and the light intensity distribution SP2(C)of the light spot for optical scanning along the scanning line SL2 ismatched with the position of the scanning line SL0 as shown in FIG. 14D.At the position D, exposure is performed with the light intensitydistribution SP1(D) of the light spot for optical scanning along thescanning line SL1 as shown in FIG. 14E.

Thus, the substantially straight scanning line SL0 can be realized byusing one or combined two of the light spots for optical scanning alongthe three scanning lines SL1, SL2, and SL3. The correction can also beperformed by using one or combined two of light spots for opticalscanning along four or more scanning lines.

Thereby, the position of the center of gravity of an exposuredistribution for forming a dot line extending in the main scanningdirection can be shifted over a plurality of scanning lines (thescanning lines SL1 and SL2 in FIG. 14A) adjacent in the sub scanningdirection.

This means the following. That is, letting the scanning lines adjacentin the sub scanning direction be scanning lines i−1, i, and i+1, thecenter of gravity of the exposure distribution formed by the light spoton the scanning line i−1 and the light spot on the scanning line i canbe shifted in position between the scanning lines i−1 and i byrelatively adjusting the light intensities of the light spots. Further,the center of gravity of the exposure distribution formed by the lightspot on the scanning line i and the light spot on the scanning line i+1can be shifted in position between the scanning lines i and i+1 byrelatively adjusting the light intensities of the light spots.

Thus, by adjusting the light intensities of adjacent two of light spotsformed on three or more scanning lines while writing one of the scanninglines, the center of gravity of the exposure distribution may be shiftedbetween two or more of the scanning lines (the scanning lines i−1, i,and i+1 in the above-described case). This is the case where the centerof gravity of an exposure distribution is shifted over a plurality ofscanning lines.

In the above-described case, the yellow scanning line has its scanningline curving and its position in the sub scanning direction corrected soas to match the black scanning line. In the very same manner asdescribed above, the magenta scanning line and the cyan scanning linecan be corrected so as to coincide with the black scanning line.

That is, in at least one of the scanning regions (in this embodiment, ineach of the photosensitive bodies 8A′, 8A, and 8B of the photosensitivebodies 8A′, 8A, 8B, and 8B′) that are provided in the sub scanningdirection, an exposure distribution may be formed to have a center ofgravity between scanning lines adjacent in the sub scanning direction.The position of the center of gravity in the sub scanning direction maybe shifted. Thereby, the optical scanning positions in the sub scanningdirection in the scanning regions may be corrected relative to eachother. In this case, the center of gravity of the exposure distributioncan also be shifted over a plurality of scanning lines. Further,scanning line curving in one scanning region may be corrected relativeto scanning line curving in another scanning region.

The above-described arrangement of a plurality of scanning regions inthe sub scanning direction may be realized, for instance, by arrangingphotoconductive photosensitive bodies as the scanning regions in the subscanning direction so that the photosensitive bodies extend in the mainscanning direction parallel to each other. Therefore, the opticalscanner of the fourth embodiment may be used for a tandem-type imagingapparatus. The scanning regions arranged in the sub scanning directionmay also be different optical scanning positions on the samephotosensitive surface of a photosensitive body at which opticalscanning positions different images are written to the photosensitivesurface.

The above-described relative correction of the optical scanningpositions in the sub scanning direction in the scanning regions is toadjust the relative positions of the scanning lines in the sub scanningdirection in the scanning regions at the stage where the scanning linecurving of each scanning line is corrected. This may be realized bycontrolling the formation of an exposure distribution. As previouslydescribed, however, the adjustment of the relative positions of thescanning lines in the sub scanning direction can be further facilitatedby making the position of the scanning line adjustable in the subscanning direction in at least one of the optical scanning parts byadjusting the modulation timing of the light source.

This adjustment is realized as follows. In the case of considering arotary polygonal mirror having n deflecting and reflecting surfaces asthe optical deflection and scanning part, when the rotary polygonalmirror is rotated so that each deflecting and reflecting surfacedeflects a light beam, n scanning lines can be formed by one rotation ofthe rotary polygonal mirror. For instance, in the case of n=6, onerotation of the rotary polygonal mirror can form six scanning lines.

Such a rotary polygonal mirror is employed, and its deflecting andreflecting surfaces are numbered 1 through 6 so that the odd-numbereddeflecting and reflecting surfaces deflect a light beam. Thereby, threescanning lines are formed by every rotation of the rotary polygonalmirror. At this point, the modulation timing of the light beam isadjusted so that the even-numbered deflecting and reflecting surfacesdeflect a light beam. Thereby, the positions of the scanning lines canbe shifted by half of the scanning line pitch in the sub scanningdirection from those of the scanning lines formed by the light beamdeflected by the odd-numbered deflecting and reflecting surfaces.

Generally, in the case of deflecting a light beam by the first one ofevery N deflecting and reflecting surfaces, when the modulation timingof the light source is adjusted so that the light beam is deflected bythe deflecting and reflecting surface next to the first one of every Ndeflecting and reflecting surfaces, the positions of the scanning linescan be shifted by one N^(th) of the scanning line pitch in the subscanning direction. Therefore, if N is suitably set to a large number,the scanning line position can be shifted finely in the sub scanningdirection.

When N increases, however, the number of optical scans per rotation ofthe rotary polygonal mirror decreases, thus making it difficult torealize high-speed optical scanning. Generally, therefore, N is suitablyset to two or three. In this case, the scanning ling position isadjusted by half or one third of the scanning line pitch, although thismuch of adjustment of the scanning line position is not necessarilysufficient. However, by combining this adjustment with the methodemployed by the optical scanner of the fourth embodiment, by whichmethod an exposure distribution is formed to have a center of gravitybetween scanning lines adjacent in the sub scanning direction, and theposition of the center of gravity in the sub scanning direction can beshifted, the position and the curve (including a tilt) of a scanningline can be corrected precisely.

The above description is given, with reference to FIGS. 2A and 2B, ofthe optical scanner according to the fourth embodiment of the presentinvention. The scanning line correction according to the fourthembodiment, however, may also be realized by the imaging apparatus ofFIG. 11 according to the third embodiment of the present invention. Theimaging apparatus of FIG. 11, to which the scanning line correctionaccording to the fourth embodiment of the present invention is applied,includes the optical scanner according to the fourth embodiment foroptically scanning the photoconductive photosensitive bodies 150Y, 150M,150C, and 150K as the scanning regions. Electrostatic latent images areformed on the photosensitive bodies 150Y, 150M, 150C, and 150K byoptical scanning to be made visible by the corresponding different colortoners. The color toner images are transferred to the sheet-likerecording medium P superimposed one over the other, thereby forming acomposite color image.

Since the color toner images are transferred to the sheet-like recordingmedium P superimposed one over the other, the photosensitive bodies150Y, 150M, 150C, and 150K substantially forming the scanning regions onwhich the electrostatic latent images are formed are arranged in the subscanning direction. The formed composite image is, for instance, atwo-color image or a multi-color image.

One of the toners for making visible the electrostatic latent imagesformed on the respective scanning regions may be a black toner, and theelectrostatic latent image formed by forming the exposure distributionhaving the center of gravity between the scanning lines adjacent in thesub scanning direction may be made visible by any of the color tonersother than the black toner.

In the case of forming a color image in a tandem-type imaging apparatus,normally, black, cyan, yellow, and magenta are used as the colors of thetoner images forming the color image. In this case, in the quality ofthe color image, it is the black toner image that most requiresresolution. This is because black is by far the most frequently usedcolor for a character image requiring resolution.

When color other than black is used for a character image, it is lesslikely that only one color, for instance, cyan, is used, but in mostcases, a plurality of color toners are superimposed. In this case, inorder to improve resolution, it is of the greatest importance to reducecolor misregistration.

In the case of forming an electrostatic latent image for the black tonerimage on the above-described scanning region where an exposuredistribution is formed to have a center of gravity between scanninglines adjacent in the sub scanning direction, the actual resolution maybe slightly lower than that required by the black toner image due to thecorrection of scanning lines. Accordingly, it is preferable that anelectrostatic latent image made visible with a color toner other than ablack toner be formed in the scanning region where the exposuredistribution is formed.

Fifth Embodiment

FIG. 15 is a diagram showing an optical scanner according to a fifthembodiment of the present invention. The optical scanner of the fifthembodiment includes a photoconductive photosensitive body 48 forming asubstantial scanning surface. The photosensitive body 48 is divided intotwo scanning regions 48A and 48B in the main scanning direction or inthe direction of the axis of rotation. That is, the two scanning regions48A and 48B are arranged in the main scanning direction. The scanningregions 48A and 48B are optically scanned by optical scanning parts 50Aand 50B.

According to the optical scanner of FIG. 15, the optical scanning part50A (50B) deflects a light beam emitted from a modulated light source41A (41B) such as a semiconductor laser by an optical deflection andscanning part (polygon mirror) 45A (45B) so that the deflected lightbeam is guided by a scanning and imaging optical system 46A (46B) to beformed into a light spot on the scanning region 48A (48B). The scanningregions 48A and 48B of the scanning surface 48 are optically scanned bythe respective light spots so that a plurality of images to form adesired image are written separately to the scanning regions 48A and48B.

The optical scanning part 50A (50B) includes a coupling lens 42A (42B)coupling the light beam from the light source 41A (41B), an aperture 43A(43B) for beam shaping, a cylindrical lens 44A (44B), a mirror foroptical path bending 47A (47B), and a photosensor 49A (49B).

In the optical scanning part 50A (50B), the light beam emitted from thelight source 41A (41B) is coupled by the coupling lens 42A (42B) to beshaped by the aperture 43A (43B). Then, the light beam is focused by thecylindrical lens 44A (44B) on a deflecting and reflecting surface of thepolygon mirror 45A (45B) as a line image extending in the main scanningdirection. The light beam deflected by the polygon mirror 45A (45B) isfocused into the light spot on the scanning region 48A (48B) by theaction of the scanning and imaging optical system 46A (46B).

The optical paths of the light beams passing through the scanning andimaging optical systems 46A and 46B are bent by the mirrors 47A and 47Bto be detected by the photosensors 49A and 49B, respectively.Synchronization is established between the optical scans based on thedetection results.

In at least one of the scanning regions 48A and 48B, an exposuredistribution may be formed to have a center of gravity between scanninglines adjacent in the sub scanning direction, and the position of thecenter of gravity in the sub scanning direction may be movable.

For instance, the optical scanning part 50A has the function of formingan exposure distribution having a center of gravity between scanninglines adjacent in the sub scanning direction in the scanning region 50Aand varying the position of the center of gravity. In this case, whenneither of scanning lines SLA1 and SLA2 in the scanning region 48A isaligned with the corresponding scanning line SLB in the scanning region48B in the main scanning direction (or when there is a deviation in thesub scanning direction between the scanning line SLB and each of thescanning lines SLA1 and SLA2) as shown in FIG. 16A, an exposuredistribution may be formed between the scanning lines SLA1 and SLA2 sothat the scanning line SLA0 formed of the trace of the center of gravityof the exposure distribution is aligned with the scanning line SLB.

That is, in the scanning region 48A, which is at least one of thescanning regions, an exposure distribution is formed to have a center ofgravity between the scanning lines SLA1 and SLA2 adjacent in the subscanning direction. The position of the center of gravity in the subscanning direction is shifted so that the optical scanning positions inthe sub scanning direction in the scanning regions 48A and 48B may becorrected relative to each other.

According to the optical scanner of the fifth embodiment, the scanningregions are arranged in the main scanning direction. That is, all theregion optically scanned to obtain a desired image is divided into thescanning regions in the main scanning direction, and the scanningregions are optically scanned by the respective different scanningparts. The desired image is formed by connecting the partial imageswritten to the individual scanning regions in the main scanningdirection. In this case, if there is a misalignment of scanning lines ora difference in the degree of scanning line curving between the scanningregions, the image quality is significantly degraded.

According to this optical scanner, the position of the center of gravityof the exposure distribution is adjustable in at least one of thescanning regions, so that the misalignment of scanning lines or thedifference in the degree of scanning line curving between the one of thescanning regions and a scanning region adjacent thereto can be reducedeffectively.

Further, scanning line curving in the scanning region 48A and scanningline curving in the scanning region 48B may also be corrected relativeto each other. This is apparent from the above description based onFIGS. 12A through 14E.

In at least one of the optical scanning parts 50A and 50B of the opticalscanner of FIG. 15, the scanning line position may be adjusted in thesub scanning direction by adjusting the modulation timing of the lightsource 41A or 41B. By combining the adjustment of the scanning lineposition by the adjustment of the modulation timing with the correctionbased on the shift of the center of gravity position of the exposuredistribution, scanning line curving can be corrected more easily.

Alternatively, when the scanning lines SLA1 and SLA2 in the scanningregion 48A are inclined differently from scanning lines SLB1 and SLB2 inthe scanning region 48B as shown in FIG. 16B, the scanning line SLA0 andthe corresponding scanning SLB0 may be formed in the scanning regions48A and 48B, respectively, so as to be aligned with each other byperforming correction based on the exposure distribution in each of thescanning regions 48A and 48B.

An imaging apparatus according to the fifth embodiment of the presentinvention includes the optical scanner of the fifth embodiment.According to the imaging apparatus of the fifth embodiment, each of thescanning regions 48A and 48B is a photoconductive photosensitive body,and the optical scanner optically scans the scanning regions 48A and 48Bso that electrostatic latent images are formed on the scanning regions48A and 48B. The electrostatic latent images are made visible with tonerso as to be transferred compositely to a sheet-like recording medium,thereby forming a composite image.

The imaging apparatus may include a plurality of scanning surfacesarranged in the sub scanning direction, the scanning surfaces eachincluding a plurality of scanning regions in the main scanningdirection. Electrostatic latent images formed on the respective scanningsurfaces may be made visible with different colors so that the visiblecolor images are superimposed to form a multi-color image.

In the optical scanner of FIG. 15, the two scanning regions 48A and 48Bare optically scanned by the two optical scanning parts 50A and 50B.However, the optical scanner may be configured so that three or morescanning regions may be optically scanned by as many optical scanningparts. When a plurality of scanning regions are thus provided in themain scanning direction so that the scanning regions are opticallyscanned independently by different optical scanning parts, a large-sizeimage can be formed by increasing the size of the scanning surface inthe main scanning direction.

Further, by increasing the number of optical scanning parts so as toreduce the size of each individual scanning region in the main scanningdirection, the angle of view of each of the optical scanning partsoptically scanning the individual scanning regions can be reduced. Thismakes it easier to improve the optical performance of the opticalscanning parts, so that an excellent image can be formed.

In the case of dividing the scanning surface into a plurality ofscanning regions and optically scanning the scanning regionsindependently of each other so that a composite image is formed bycombining the images formed on the respective scanning regions, scanningline curving or scanning line tilting, which does not stand out in theindividual image formed on each scanning region, stands outsignificantly in the composite image if there is a misalignment ofscanning lines as shown in FIG. 16A or discontinuity in the inclinationof scanning lines as shown in FIG. 16B, thereby greatly degrading thequality of the composite image. However, by correcting the position inthe sub scanning direction, the tilt, or the curvature of a scanningline as described above, an excellent image can be obtained.

When the scanning regions 48A and 48B shown in FIG. 16A or 16B aredefined as one unit, and the four same units are arranged in the subscanning direction, a large-size composite color image can be obtainedby making visible electrostatic latent images formed on the respectiveunits (photosensitive bodies) with toners of different colors,transferring the visible images to a sheet-like recording medium, andfixing the transferred images.

In the above-described fifth embodiment, the scanning regions arearranged in the main scanning direction. It is apparent, however, thatthe above-described scanning line correction is also applicable in thecase of a single scanning region. That is, in the optical scanner ofFIG. 15, the optical scanning part 50A and the scanning region 48A maybe separated from the optical scanner to form a conventional opticalscanner.

Such an optical scanner performing the above-described scanning linecorrection includes an optical scanning part in which a light beamemitted from a modulated light source is deflected by an opticaldeflection and scanning part so that the deflected light beam is guidedby a scanning and imaging optical system to be formed into a light spoton a scanning surface, the light spot optically scanning the scanningsurface so that a desired image is written to the scanning surface. Inthe scanning region, an exposure distribution is formed to have a centerof gravity between scanning lines adjacent in the sub scanning directionso that the position of the center of gravity in the sub scanningdirection can be shifted.

According to this optical scanner, when a dot line is formed in thescanning region so as to extend in the main scanning direction, thedistance between one of the adjacent scanning lines and the center ofgravity of the exposure distribution in the sub scanning direction maybe varied according to the position in the main scanning direction.Further, the distance between the one of the adjacent scanning lines andthe center of gravity of the exposure distribution in the sub scanningdirection may be varied continuously according to the position in themain scanning direction. Furthermore, the position of the center ofgravity of the exposure distribution for forming the dot line extendingin the main scanning direction may be shifted over a plurality ofscanning lines adjacent in the sub scanning direction.

This optical scanner may form an imaging apparatus. Such an imagingapparatus includes a photoconductive photosensitive body as the scanningregion. The optical scanner optically scans the photosensitive body(scanning region) so that an electrostatic latent image is formedthereon. The electrostatic latent image is made visible with toner to betransferred to a sheet-like recording medium. Thereby, an image isformed on the sheet-like recording medium.

The above-described optical scanners and imaging apparatuses of thefourth and fifth embodiments may also employ the above-described methodof correcting the constant velocity characteristic such as the fθcharacteristic.

Thus, according to the optical scanners of the present invention,scanning line curving and a scanning position in the main scanningdirection may be corrected and adjusted simply and easily, so thatexcellent optical scanning can be performed. The imaging apparatusesincluding the optical scanners of the present invention can form anexcellent image.

Particularly, the optical scanner according to the fourth embodiment ofthe present invention can correct scanning line curving easily withcertainty by forming an exposure distribution. A tandem-type color-imageforming apparatus including this optical scanner can form an excellentimage without color unevenness or color misregistration.

In the fourth and fifth embodiments, the two light spots forming anexposure distribution may be formed simultaneously as two light spotsadjacent in the sub scanning direction. Alternatively, the two lightspots may be formed at an interval by successive optical scans by thesame light beam so as to be adjacent in the sub scanning direction.

This is because generally, the photosensitivity of a scanning surface isthe scalar sum of exposures when the scanning surface is exposed tolight (or optically scanned) a plurality of times.

Further, the multi-beam method as well as the single-beam method can beapplied to the optical scanners and the imaging apparatuses of thefourth and fifth embodiments.

The present invention is not limited to the specifically disclosedembodiments, but variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority applications No.2002-055517 filed on Mar. 1, 2002 and No. 2002-295220 filed on Oct. 8,2002, the entire contents of which are hereby incorporated by reference.

1. An imaging apparatus comprising: N (≧2) photoconductivephotosensitive bodies arranged along a path for conveying a sheet-likerecording medium; and N optical scanners optically scanning said Nphotosensitive bodies, respectively, so as to write and form differentelectrostatic latent images thereon, the electrostatic latent imagesbeing made visible as toner images of different colors so that the tonerimages are transferred to and fixed on the sheet-like recording medium,being superimposed one over another, wherein: one of said Nphotosensitive bodies has a black toner image formed thereon; and eachof the optical scanners for the remaining N-1 photosensitive bodiescomprises: a light source modulated based on image data; an opticaldeflection and scanning part deflecting a light beam emitted from saidlight source; and a scanning and imaging optical system condensing thedeflected light beam toward a scanning surface of the correspondingphotosensitive body so as to form a light spot on the scanning surface,the light spot optically scanning the scanning surface, an effectivescanning region of the scanning surface being divided into a pluralityof regions according to a scanning line curving characteristic; andsuitable image data for optically scanning the divided regions beingselected from image data of a plurality of image lines every time thelight spot optically scans the effective scanning region, so that theimage data of each of the image lines is written with scanning linecurving being corrected based on the black toner image.
 2. The imagingapparatus as claimed in claim 1, wherein each of the optical scannersfor the remaining N-1 photosensitive bodies further corrects a positionof the light spot in a main scanning direction based on the black tonerimage.
 3. The imaging apparatus as claimed in claim 1, wherein, in eachof the optical scanners for the remaining N-1 photosensitive bodies: thelight source emits a plurality of light beams so that a plurality oflight spots are formed on the scanning surface so as to be adjacent toeach other in a sub scanning direction; and a position of a center ofgravity of a composite light intensity distribution formed bysuperimposing light intensity distributions of the light beams in thesub scanning direction is shiftable by adjusting intensity of each ofthe light beams so that the scanning line curving included in each ofthe image lines is finely adjusted to be corrected.
 4. The imagingapparatus as claimed in claim 1, wherein each of the optical scannersfor the remaining N-1 photosensitive bodies further comprises adeflection part capable of deflecting the light beam emitted from thelight source, and in each of the optical scanners for the remaining N-1photosensitive bodies, a position of the light spot on the scanningsurface is shiftable in a sub scanning direction by deflecting the lightbeam by said deflection part so that the scanning line curving includedin each of the image lines is finely adjusted to be corrected.
 5. Theimaging apparatus as claimed in claim 4, wherein said deflection partcomprises at least one liquid crystal deflection element in each of theoptical scanners for the remaining N-1 photosensitive bodies.
 6. Theimaging apparatus as claimed in claim 4, wherein said deflection partcomprises a deflection mirror having a reflecting surface whoseorientation is adjustable in each of the optical scanners for theremaining N-1 photosensitive bodies.
 7. The imaging apparatus as claimedin claim 1, wherein, in each of the optical scanners for the remainingN-1 photosensitive bodies, a phase of a pixel clock signal of image datato be written is shifted based on a clock signal whose frequency ishigher than that of the pixel clock signal so that a position of thelight spot is correctively adjusted in a main scanning direction.
 8. Theimaging apparatus as claimed in claim 1, wherein each of the opticalscanners for the remaining N-1 photosensitive bodies further comprises ascanning position detecting part detecting a scanning position of thelight spot of the deflected light beam.
 9. The imaging apparatus asclaimed in claim 8, wherein said scanning position detecting partcomprises a plurality of optical sensors arranged in a main scanningdirection in each of the optical scanners for the remaining N-1photosensitive bodies.
 10. An imaging apparatus comprising: N (≧2)photoconductive photosensitive bodies arranged along a path forconveying a sheet-like recording medium; and N optical scannersoptically scanning said N photosensitive bodies, respectively, so as towrite and form different electrostatic latent images thereon, theelectrostatic latent images being made visible as toner images ofdifferent colors so that the toner images are transferred to and fixedon the sheet-like recording medium, being superimposed one over another,wherein: one of said N photosensitive bodies has a black toner imageformed thereon; and each of the optical scanners for the remaining N-1photosensitive bodies comprises: a light source modulated based on imagedata to emit a plurality of light beams; an optical deflection andscanning part deflecting the light beams; and a scanning and imagingoptical system condensing the deflected light beams toward a scanningsurface of the corresponding photosensitive body so that light spots areformed on the scanning surface so as to be separated from each other ina sub scanning direction, the light spots having substantially equalscanning line curving characteristics and optically scanning thescanning surface simultaneously, an effective scanning region of thescanning surface being divided into a plurality of regions according tothe scanning line curving characteristics; and a suitable one of thelight beams being selected for optical scanning in each of the dividedregions with respect to each of image lines every time the light spotsoptically scan the effective scanning region simultaneously, so that theimage data of each of the image lines is written with scanning linecurving being corrected based on the black toner image.
 11. The imagingapparatus as claimed in claim 10, wherein each of the optical scannersfor the remaining N-1 photosensitive bodies further corrects a positionof the light spot in a main scanning direction based on the black tonerimage.
 12. An optical scanner comprising: a plurality of opticalscanning parts optically scanning a plurality of scanning regions,respectively, so as to write images to form a desired image separatelyin the scanning regions, the optical scanning parts each deflecting alight beam emitted from a modulated light source by an opticaldeflection and scanning part so that the deflected light beam is guidedby a scanning and imaging optical system to form a light spot in thecorresponding scanning region, the light spot optically scanning thecorresponding scanning region, wherein, in at least one of the scanningregions, an exposure distribution is formed to have a center of gravitybetween scanning lines adjacent in a sub scanning direction so that aposition of the center of gravity in the sub scanning direction isshiftable.
 13. The optical scanner as claimed in claim 12, wherein, inthe one of the scanning regions where the exposure distribution isformed between the scanning lines adjacent in the sub scanningdirection, a distance between one of the adjacent scanning lines and thecenter of gravity of the exposure distribution in the sub scanningdirection varies according to a position in a main scanning direction ata time of forming a dot line extending in the main scanning direction.14. The optical scanner as claimed in claim 13, wherein, in the one ofthe scanning regions where the exposure distribution is formed betweenthe scanning lines adjacent in the sub scanning direction, the distancebetween the one of the adjacent scanning lines and the center of gravityof the exposure distribution in the sub scanning direction variescontinuously according to the position in the main scanning direction atthe time of forming the dot line extending in the main scanningdirection.
 15. The optical scanner as claimed in claim 13, wherein theposition of the center of gravity of the exposure distribution forforming the dot line extending in the main scanning direction isshiftable over a plurality of scanning lines adjacent in the subscanning direction.
 16. The optical scanner as claimed in claim 12,wherein: the scanning regions are arranged in the sub scanningdirection; and an exposure distribution is formed to have a center ofgravity between scanning lines adjacent in the sub scanning direction inat least one of the scanning regions so as to shift a position of thecenter of gravity in the sub scanning direction so that optical scanningpositions in the scanning regions are corrected relative to each otherin the sub scanning direction.
 17. The optical scanner as claimed inclaim 12, wherein: the scanning regions are arranged in the sub scanningdirection; and an exposure distribution is formed to have a center ofgravity between scanning lines adjacent in the sub scanning direction inat least one of the scanning regions so as to shift a position of thecenter of gravity in the sub scanning direction so that scanning linecurvings in the scanning regions are corrected relative to each other.18. The optical scanner as claimed in claim 12, wherein, in at least oneof said optical scanning parts, a position of a scanning line isadjustable in the sub scanning direction by controlling modulationtiming of the light source.
 19. The optical scanner as claimed in claim12, wherein: the scanning regions are arranged in a main scanningdirection; and an exposure distribution is formed to have a center ofgravity between scanning lines adjacent in the sub scanning direction inat least one of the scanning regions so as to shift a position of thecenter of gravity in the sub scanning direction so that optical scanningpositions in the scanning regions are corrected relative to each otherin the sub scanning direction.
 20. The optical scanner as claimed inclaim 19, wherein, in at least one of said optical scanning parts, aposition of a scanning line is adjustable in the sub scanning directionby controlling modulation timing of the light source.
 21. The opticalscanner as claimed in claim 12, wherein: the scanning regions arearranged in a main scanning direction; and an exposure distribution isformed to have a center of gravity between scanning lines adjacent inthe sub scanning direction in at least one of the scanning regions so asto shift a position of the center of gravity in the sub scanningdirection so that scanning line curvings in the scanning regions arecorrected relative to each other.
 22. The optical scanner as claimed inclaim 21, wherein, in at least one of said optical scanning parts, aposition of a scanning line is adjustable in the sub scanning directionby controlling modulation timing of the light source.
 23. An imagingapparatus comprising: a plurality of photoconductive photosensitivebodies as scanning regions; and an optical scanner optically scanningthe scanning regions so as to form electrostatic latent images therein,the electrostatic latent images being made visible with toners ofdifferent colors as toner images, the toner images being transferred toa sheet-like recording medium superimposed one over another so as toform a desired image, wherein: said optical scanner comprises aplurality of optical scanning parts optically scanning the scanningregions, respectively, so as to write the electrostatic latent images toform the desired image separately in the scanning regions, the opticalscanning parts each deflecting a light beam emitted from a modulatedlight source by an optical deflection and scanning part so that thedeflected light beam is guided by a scanning and imaging optical systemto form a light spot in the corresponding scanning region, the lightspot optically scanning the corresponding scanning region; and anexposure distribution is formed to have a center of gravity betweenscanning lines adjacent in a sub scanning direction in at least one ofthe scanning regions so that a position of the center of gravity in thesub scanning direction is shiftable.
 24. The imaging apparatus asclaimed in claim 23, wherein: one of the toners for making visible theelectrostatic latent images formed in the scanning regions is a blacktoner; and any one of the electrostatic latent images formed by formingthe exposure distribution having the center of gravity between thescanning lines adjacent in the sub scanning direction is made visiblewith one of the toners other than the black toner.
 25. The imagingapparatus as claimed in claim 23, wherein, in the one of the scanningregions where the exposure distribution is formed between the scanninglines adjacent in the sub scanning direction, a distance between one ofthe adjacent scanning lines and the center of gravity of the exposuredistribution in the sub scanning direction varies according to aposition in a main scanning direction at a time of forming a dot lineextending in the main scanning direction.
 26. The imaging apparatus asclaimed in claim 25, wherein in the one of the scanning regions wherethe exposure distribution is formed between the scanning lines adjacentin the sub scanning direction, the distance between the one of theadjacent scanning lines and the center of gravity of the exposuredistribution in the sub scanning direction varies continuously accordingto the position in a main scanning direction at the time of forming thedot line extending in the main scanning direction.
 27. The imagingapparatus as claimed in claim 25, wherein the position of the center ofgravity of the exposure distribution for forming the dot line extendingin the main scanning direction is shiftable over a plurality of scanninglines adjacent in the sub scanning direction.
 28. The imaging apparatusas claimed in claim 23, wherein: the scanning regions are arranged inthe sub scanning direction; and an exposure distribution is formed tohave a center of gravity between scanning lines adjacent in the subscanning direction in at least one of the scanning regions so as toshift a position of the center of gravity in the sub scanning directionso that optical scanning positions in the scanning regions are correctedrelative to each other in the sub scanning direction.
 29. The imagingapparatus as claimed in claim 23, wherein: the scanning regions arearranged in the sub scanning direction; and an exposure distribution isformed to have a center of gravity between scanning lines adjacent inthe sub scanning direction in at least one of the scanning regions so asto shift a position of the center of gravity in the sub scanningdirection so that scanning line curvings in the scanning regions arecorrected relative to each other.
 30. The imaging apparatus as claimedin claim 23, wherein, in at least one of the optical scanning parts ofsaid optical scanner, a position of a scanning line is adjustable in thesub scanning direction by controlling modulation timing of the lightsource.
 31. The imaging apparatus as claimed in claim 23, wherein: thescanning regions are arranged in a main scanning direction; and anexposure distribution is formed to have a center of gravity betweenscanning lines adjacent in the sub scanning direction in at least one ofthe scanning regions so as to shift a position of the center of gravityin the sub scanning direction so that optical scanning positions in thescanning regions are corrected relative to each other in the subscanning direction.
 32. The imaging apparatus as claimed in claim 31,wherein, in at least one of the optical scanning parts of said opticalscanner, a position of a scanning line is adjustable in the sub scanningdirection by controlling modulation timing of the light source.
 33. Theimaging apparatus as claimed in claim 23, wherein: the scanning regionsare arranged in a main scanning direction; and an exposure distributionis formed to have a center of gravity between scanning lines adjacent inthe sub scanning direction in at least one of the scanning regions so asto shift a position of the center of gravity in the sub scanningdirection so that scanning line curvings in the scanning regions arecorrected relative to each other.
 34. The imaging apparatus as claimedin claim 33, wherein, in at least one of the optical scanning parts ofsaid optical scanner, a position of a scanning line is adjustable in thesub scanning direction by controlling modulation timing of the lightsource.
 35. An optical scanner comprising: an optical scanning partoptically scanning a scanning region so as to write a desired imagetherein, the optical scanning part deflecting a light beam emitted froma modulated light source by an optical deflection and scanning part sothat the deflected light beam is guided by a scanning and imagingoptical system to form a light spot in the scanning region, the lightspot optically scanning the scanning region, wherein an exposuredistribution is formed to have a center of gravity between scanninglines adjacent in a sub scanning direction in the scanning region sothat a position of the center of gravity in the sub scanning directionis shiftable.
 36. The optical scanner as claimed in claim 35, wherein adistance between one of the adjacent scanning lines and the center ofgravity of the exposure distribution in the sub scanning directionvaries according to a position in a main scanning direction at a time offorming a dot line extending in the main scanning direction in thescanning region.
 37. The optical scanner as claimed in claim 36, whereinthe distance between the one of the adjacent scanning lines and thecenter of gravity of the exposure distribution in the sub scanningdirection varies continuously according to the position in the mainscanning direction at the time of forming the dot line extending in themain scanning direction in the scanning region.
 38. The optical scanneras claimed in claim 36, wherein the position of the center of gravity ofthe exposure distribution for forming the dot line extending in the mainscanning direction is shiftable over a plurality of scanning linesadjacent in the sub scanning direction.
 39. An imaging apparatuscomprising: a photoconductive photosensitive body as a scanning region;and an optical scanner optically scanning the scanning region so as toform an electrostatic latent image therein, the electrostatic latentimage being made visible with toner as a toner image, the toner imagebeing transferred to a sheet-like recording medium so as to form adesired image, wherein: said optical scanner comprises an opticalscanning part optically scanning the scanning region so as to write thedesired image therein, the optical scanning part deflecting a light beamemitted from a modulated light source by an optical deflection andscanning part so that the deflected light beam is guided by a scanningand imaging optical system to form a light spot in the scanning region,the light spot optically scanning the scanning region; and an exposuredistribution is formed to have a center of gravity between scanninglines adjacent in a sub scanning direction in the scanning region sothat a position of the center of gravity in the sub scanning directionis shiftable.
 40. The imaging apparatus as claimed in claim 39, whereina distance between one of the adjacent scanning lines and the center ofgravity of the exposure distribution in the sub scanning directionvaries according to a position in a main scanning direction at a time offorming a dot line extending in the main scanning direction in thescanning region.
 41. The imaging apparatus as claimed in claim 40,wherein the distance between the one of the adjacent scanning lines andthe center of gravity of the exposure distribution in the sub scanningdirection varies continuously according to the position in the mainscanning direction at the time of forming the dot line extending in themain scanning direction in the scanning region.
 42. The imagingapparatus as claimed in claim 40, wherein the position of the center ofgravity of the exposure distribution for forming the dot line extendingin the main scanning direction is shiftable over a plurality of scanninglines adjacent in the sub scanning direction.